<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

# 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)

...

return 'arn:aws:%s:%s:%s:%s' % (service, aws_stack.get_region(), TEST_AWS_ACCOUNT_ID, resource)
def send_describe_dynamodb_ttl_request(table_name):
    return send_dynamodb_request('', 'DescribeTimeToLive', json.dumps({'TableName': table_name}))
def send_update_dynamodb_ttl_request(table_name, ttl_status):
    return send_dynamodb_request('', 'UpdateTimeToLive', json.dumps({
        'TableName': table_name,
        'TimeToLiveSpecification': {
            'AttributeName': 'ExpireItem',
            'Enabled': ttl_status
        }
    }))
def send_dynamodb_request(path, action, request_body):
    headers = {
        'Host': 'dynamodb.amazonaws.com',
        'x-amz-target': 'DynamoDB_20120810.{}'.format(action),
        'authorization': 'some_token'
    }
    url = '{}/{}'.format(os.getenv('TEST_DYNAMODB_URL'), path)
    return requests.put(url, data=request_body, headers=headers, verify=False)
def create_sqs_queue(queue_name):
    """Utility method to create a new queue via SQS API"""
    client = aws_stack.connect_to_service('sqs')
    
    queue_url = client.create_queue(QueueName=queue_name)['QueueUrl']
    
    queue_arn = client.get_queue_attributes(
        QueueUrl=queue_url,
        AttributeNames=['QueueArn'],
    )['Attributes']['QueueArn']