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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)

...

region_name = extract_region_from_arn(queue_arn)
    client = connect_to_service('sqs', region_name=region_name)
    queue_url = get_sqs_queue_url(queue_arn)
    response = client.receive_message(QueueUrl=queue_url)
    return response
def firehose_name(firehose_arn):
    return firehose_arn.split('/')[-1]
def mock_aws_request_headers(service='dynamodb', region_name=None):
    ctype = APPLICATION_AMZ_JSON_1_0
    if service == 'kinesis':
        ctype = APPLICATION_AMZ_JSON_1_1
    elif service in ['sns', 'sqs']:
        ctype = APPLICATION_X_WWW_FORM_URLENCODED
    access_key = get_boto3_credentials().access_key
    region_name = region_name or get_region()
    headers = {
        'Content-Type': ctype,
        'Accept-Encoding': 'identity',
        'X-Amz-Date': '20160623T103251Z',
        'Authorization': ('AWS4-HMAC-SHA256 ' +
            'Credential=%s/20160623/%s/%s/aws4_request, ' +
            'SignedHeaders=content-type;host;x-amz-date;x-amz-target, Signature=1234') % (
                access_key, region_name, service)
    }
    return headers
def dynamodb_get_item_raw(request):