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# Type of request: This is a ...

[x] bug report
[ ] feature request

# Detailed description

Use localstack as AWS kinesis service mock and try to put a record with raw string data

```python
client.put_record(StreamName=stream_name, Data="raw_string", PartitionKey="MyParticonKey")
```

Then get the record using `subscribe_to_shard`

```python
result = self._client.subscribe_to_shard(
       ConsumerARN=self._consumer_arn, ShardId='shardId-000000000000', StartingPosition=start_position)
event = next(event_stream_iterator.__iter__())
```

And there is a `binascii.Error: Incorrect padding`. Backtrace:

```python
> /root/watercube/ms/aws/kds_consumer.py(188)_handle_event_stream()                                                                                                                                                                                           [0/1969]
-> event = next(event_stream_iterator.__iter__())
  /usr/local/lib/python3.7/site-packages/botocore/eventstream.py(571)__iter__()
-> parsed_event = self._parse_event(event)
  /usr/local/lib/python3.7/site-packages/botocore/eventstream.py(584)_parse_event()
-> parsed_response = self._parser.parse(response_dict, self._output_shape)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(242)parse()
-> parsed = self._do_parse(response, shape)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(644)_do_parse()
-> final_parsed[event_type] = self._do_parse(response, event_shape)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(648)_do_parse()
-> self._parse_payload(response, shape, shape.members, final_parsed)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(689)_parse_payload()
-> body_parsed = self._parse_shape(shape, original_parsed)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(572)_handle_structure()
-> raw_value)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(310)_handle_list()
-> parsed.append(self._parse_shape(member_shape, item))
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(572)_handle_structure()
-> raw_value)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(586)_handle_blob()
-> return self._blob_parser(value)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(215)_default_blob_parser()
-> return base64.b64decode(value)
  /usr/local/lib/python3.7/base64.py(87)b64decode()
-> return binascii.a2b_base64(s)
```

While using AWS service endpoint, this does not happen.

## Expected behavior

`subscribe_to_shard` could properly decode the data returned from localstack when the record is put as string

## Actual behavior

`subcribe_to_shard` gets decode error

# Steps to reproduce

## Command used to start LocalStack

Start the localstack container using docker-compose,

```yaml
version: '2.4'
services:
    aws: 
         image: localstack/localstack:0.12.10
         restart: always
         ports: 
           - "4566"
         enviroments:
           - LOCALSTACK_SERVICES=s3,kinesis
           - LOCALSTACK_DEBUG=1
           - LOCALSTACK_HOSTNAME=aws
```

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

```python
client.put_record(StreamName=stream_name, Data="raw_string", PartitionKey="MyParticonKey")
```

```python
result = self._client.subscribe_to_shard(
       ConsumerARN=self._consumer_arn, ShardId='shardId-000000000000', StartingPosition=start_position)
event = next(event_stream_iterator.__iter__())
```
- boto3==1.9.134
- botocore==1.12.253
client = aws_stack.create_external_boto_client("kinesis")
        stream_name = "test-%s" % short_uid()
        client.create_stream(StreamName=stream_name, ShardCount=1)
        sleep(1.5)
        client.put_records(
            StreamName=stream_name,
            Records=[{"Data": "SGVsbG8gd29ybGQ=", "PartitionKey": "1"}],
        )
        
        iterator = self._get_shard_iterator(stream_name)
        response = client.get_records(ShardIterator=iterator)
        json_records = response.get("Records")
        self.assertEqual(1, len(json_records))
        self.assertIn("Data", json_records[0])
        
        iterator = self._get_shard_iterator(stream_name)
        url = config.get_edge_url()
        headers = aws_stack.mock_aws_request_headers("kinesis")
        headers["Content-Type"] = constants.APPLICATION_AMZ_CBOR_1_1
        headers["X-Amz-Target"] = "Kinesis_20131202.GetRecords"