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# Type of request: This is a ...

[x] bug report
[ ] feature request

# Detailed description
VTL request containing an expression evaluated to String Concatenation does not work.

### Query
```
mutation createShop {
            createShop(shop: { shopName: "Apple" } ) {
              shopName
            }
          }
```

### VTL Template (**Works when deploy to Appsync on AWS services**)
```
{
  "version" : "2018-05-29",
  "operation" : "PutItem",
  "key" : {
    "PK": $util.dynamodb.toDynamoDBJson("SHOP#${ctx.args.shop.shopName}"),
    "SK": $util.dynamodb.toDynamoDBJson("PROFILE#$ctx.args.shop.shopName")
  },
  "attributeValues" : { "shopName": { "S": "Apple" }}
}
```

## Expected behavior
### Item stored in dynamodb 
```
{
            "PK": {
                "S": "SHOP#Apple"
            },
            "SK": {
                "S": "PROFILE#Apple"
            },
            "shopName": {
                "S": "Apple"
            }
        }
```


## Actual behavior
### Item stored in dynamodb 
```
{
            "PK": {
                "S": "SHOP#${ctx.args.shop.shopName}"
            },
            "SK": {
                "S": "PROFILE#$ctx.args.shop.shopName"
            },
            "shopName": {
                "S": "Apple"
            }
        }
```

# Steps to reproduce

## Command used to start LocalStack
docker-compose
````
version: '2.1'

services:
  localstack:
    container_name: "${LOCALSTACK_DOCKER_NAME-localstack_main}"
    image: localstack/localstack
    ports:
      - "4566-4700:4566-4700"
      - "443:443"
      - "${PORT_WEB_UI-8080}:${PORT_WEB_UI-8080}"
    environment:
      - SERVICES=serverless,cloudformation,dynamodb,iam,s3,appsync,edge
      - DEBUG=1
      - DATA_DIR=${DATA_DIR- }
      - PORT_WEB_UI=${PORT_WEB_UI- }
      - LAMBDA_EXECUTOR=${LAMBDA_EXECUTOR- }
      - KINESIS_ERROR_PROBABILITY=${KINESIS_ERROR_PROBABILITY- }
      - DOCKER_HOST=unix:///var/run/docker.sock
      - HOST_TMP_FOLDER=${TMPDIR}
      - LOCALSTACK_API_KEY=${LOCALSTACK_API_KEY}
    volumes:
      - "${TMPDIR:-/tmp/localstack}:/tmp/localstack"
      - "/var/run/docker.sock:/var/run/docker.sock"
````

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

...

log_group_name = '/aws/lambda/%s' % func_name
        streams = logs_client.describe_log_streams(logGroupName=log_group_name)['logStreams']
        streams = sorted(streams, key=lambda x: x['creationTime'], reverse=True)
        log_events = logs_client.get_log_events(
            logGroupName=log_group_name, logStreamName=streams[0]['logStreamName'])['events']
        return log_events
class TestLambdaBaseFeatures(unittest.TestCase):
    def test_forward_to_fallback_url_dynamodb(self):
        db_table = 'lambda-records'
        ddb_client = aws_stack.connect_to_service('dynamodb')
        def num_items():
            return len((run_safe(ddb_client.scan, TableName=db_table) or {'Items': []})['Items'])
        items_before = num_items()
        _run_forward_to_fallback_url('dynamodb://%s' % db_table)
        items_after = num_items()
        self.assertEqual(items_after, items_before + 3)
    def test_forward_to_fallback_url_http(self):
        class MyUpdateListener(ProxyListener):
            def forward_request(self, method, path, data, headers):
                records.append({'data': data, 'headers': headers})
                return 200
        records = []
        local_port = get_free_tcp_port()
        proxy = start_proxy(local_port, backend_url=None, update_listener=MyUpdateListener())
        items_before = len(records)
        _run_forward_to_fallback_url('%s://localhost:%s' % (get_service_protocol(), local_port))
        items_after = len(records)
        for record in records:
            self.assertIn('non-existing-lambda', record['headers']['lambda-function-name'])
        self.assertEqual(items_after, items_before + 3)