Despite `ReturnValues` being set to `ALL_OLD`, the written attributes are returned. Expected behavior is `null` returned when the item didn't previously exist. Logs from Java client below. Actual data being written/returned has been edited.

```
01:08:34.907 [DataService-akka.actor.default-dispatcher-74] DEBUG com.amazonaws.request - Sending Request: POST http://localstack:4569 / Headers: (User-Agent: a
 Linux/4.14.131-linuxkit OpenJDK_64-Bit_Server_VM/25.212-b03 java/1.8.0_212 scala/2.12.10 dynamodb-table-api/1.11.483, amz-sdk-invocation-id: f278edab-6204-75f0
Content-Length: 1096, X-Amz-Target: DynamoDB_20120810.PutItem, Content-Type: application/x-amz-json-1.0, )
01:08:34.908 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "POST / HTTP/1.1[\r][\n]"
01:08:34.909 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "Host: localstack:4569[\r][\n]"
01:08:34.909 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "Authorization: AWS4-HMAC-SHA256 Credential=localst
0212/us-west-2/dynamodb/aws4_request, SignedHeaders=amz-sdk-invocation-id;amz-sdk-retry;content-length;content-type;host;user-agent;x-amz-date;x-amz-target, Sig
03fcb5552c91ae79c3d2939f3c1634bcfa7666b97bafd200f3d[\r][\n]"
01:08:34.909 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "X-Amz-Date: 20200212T010834Z[\r][\n]"
01:08:34.909 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "User-Agent: aws-sdk-java/1.11.483 Linux/4.14.131-l
it_Server_VM/25.212-b03 java/1.8.0_212 scala/2.12.10 dynamodb-table-api/1.11.483[\r][\n]"
01:08:34.909 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "amz-sdk-invocation-id: f278edab-6204-75f0-5fc4-a8c

01:08:34.909 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "amz-sdk-retry: 0/0/500[\r][\n]"
01:08:34.909 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "X-Amz-Target: DynamoDB_20120810.PutItem[\r][\n]"
01:08:34.910 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "Content-Type: application/x-amz-json-1.0[\r][\n]"
01:08:34.910 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "Content-Length: 1096[\r][\n]"
01:08:34.910 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "Connection: Keep-Alive[\r][\n]"
01:08:34.911 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "[\r][\n]"
01:08:34.911 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 >> "{"TableName":"my-table","Item":{"hashKey":{"S":"hk"},"rangeKey":{"S":"rk"}},"ReturnValues":"ALL_OLD","ConditionExpression":"attribute_not_exists(rangeKey)"}"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "HTTP/1.1 200 OK[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "Server: BaseHTTP/0.6 Python/3.7.5[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "Date: Wed, 12 Feb 2020 01:08:34 GMT[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "Content-Type: application/x-amz-json-1.0[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "x-amz-crc32: 1911172288[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "x-amzn-RequestId: 32a8e610-50be-40d8-9e81-107c10fb18f6[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "content-length: 868[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "Access-Control-Allow-Origin: *[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "Access-Control-Allow-Methods: HEAD,GET,PUT,POST,DELETE,OPTIONS,PATCH[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "Access-Control-Allow-Headers: authorization,content-type,content-md5,cache-control,x-amz-content-sha256,x-amz-date,x-amz-security-token,x-amz-user-agent,x-amz-target,x-amz-acl,x-amz-version-id,x-localstack-target,x-amz-tagging[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "Access-Control-Expose-Headers: x-amz-version-id[\r][\n]"
01:08:34.994 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "[\r][\n]"
01:08:35.035 [DataService-akka.actor.default-dispatcher-74] DEBUG org.apache.http.wire - http-outgoing-17 << "{"Attributes": {"hashKey": {"S": "hk"}, "rangeKey": {"S": "rk"}}}"
01:08:35.035 [DataService-akka.actor.default-dispatcher-74] DEBUG com.amazonaws.request - Received successful response: 200, AWS Request ID: 32a8e610-50be-40d8-9e81-107c10fb18f6
01:08:35.035 [DataService-akka.actor.default-dispatcher-74] DEBUG com.amazonaws.requestId - x-amzn-RequestId: 32a8e610-50be-40d8-9e81-107c10fb18f6
```

Observed with `localstack/localstack:0.10.7`. Appears to be regression from `0.9.5`.
events = [rec["eventName"] for rec in records]
        assert events == ["INSERT", "MODIFY", "REMOVE"] * 2
        # asse
        event_ids = [rec.get("eventID") for rec in records]
        assert all(event_ids)
        # asse
        for idx, record in enumerate(records):
            assert "SequenceNumber" in record["dynamodb"]
            assert record["dynamodb"]["StreamViewType"] == "KEYS_ONLY"
            assert record["dynamodb"]["Keys"] == {"Username": {"S": "Fred" if idx < 3 else "Alice"}}
            assert "OldImage" not in record["dynamodb"]
            assert "NewImage" not in record["dynamodb"]
        # cl
        delete_table(table_name)
    def test_dynamodb_with_kinesis_stream(self):
        dynamodb = aws_stack.create_external_boto_client("dynamodb")
        kinesis = aws_stack.create_external_boto_client("kinesis")
        # crea
        stream_name = "kinesis_dest_stream"
        kinesis.create_stream(StreamName=stream_name, ShardCount=1)
        # wait 
        sleep(1)
        # Get 
        stream_description = kinesis.describe_stream(StreamName=stream_name)["StreamDescription"]
        table_name = "table_with_kinesis_stream-%s" % short_uid()
        # crea
        dynamodb.create_table(
            TableName=table_name,
            KeySchema=[{"AttributeName": "Username", "KeyType": "HASH"}],
            AttributeDefinitions=[{"AttributeName": "Username", "AttributeType": "S"}],