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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)

...

proxy.stop()
    def test_adding_fallback_function_name_in_headers(self):
        lambda_client = aws_stack.connect_to_service('lambda')
        ddb_client = aws_stack.connect_to_service('dynamodb')
        db_table = 'lambda-records'
        config.LAMBDA_FALLBACK_URL = 'dynamodb://%s' % db_table
        lambda_client.invoke(FunctionName='non-existing-lambda',
                             Payload=b'{}', InvocationType='RequestResponse')
        result = run_safe(ddb_client.scan, TableName=db_table)
        self.assertEqual(result['Items'][0]['function_name']['S'], 'non-existing-lambda')
    def test_dead_letter_queue(self):
        sqs_client = aws_stack.connect_to_service('sqs')
        lambda_client = aws_stack.connect_to_service('lambda')
        
        queue_name = 'test-%s' % short_uid()
        lambda_name = 'test-%s' % short_uid()
        queue_url = sqs_client.create_queue(QueueName=queue_name)['QueueUrl']
        queue_arn = aws_stack.sqs_queue_arn(queue_name)
        testutil.create_lambda_function(
            handler_file=TEST_LAMBDA_PYTHON, func_name=lambda_name, libs=TEST_LAMBDA_LIBS,
            runtime=LAMBDA_RUNTIME_PYTHON36, DeadLetterConfig={'TargetArn': queue_arn}
        )
        
        payload = {
            lambda_integration.MSG_BODY_RAISE_ERROR_FLAG: 1
        }
        lambda_client.invoke(FunctionName=lambda_name,
                             Payload=json.dumps(payload), InvocationType='Event')
        
        def receive_dlq():