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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)

...

return {
        'QueueUrl': queue_url,
        'QueueArn': queue_arn,
    }
def get_lambda_log_group_name(function_name):
    return '/aws/lambda/{}'.format(function_name)
def get_lambda_log_events(function_name, delay_time=DEFAULT_GET_LOG_EVENTS_DELAY):
    def get_log_events(function_name, delay_time):
        time.sleep(delay_time)
        logs = aws_stack.connect_to_service('logs')
        log_group_name = get_lambda_log_group_name(function_name)
        rs = logs.filter_log_events(logGroupName=log_group_name)
        return rs['events']
    try:
        events = get_log_events(function_name, delay_time)
    except Exception as e:
        if 'ResourceNotFoundException' in str(e):
            return []
        raise
    rs = []
    for event in events:
        raw_message = event['message']
        if not raw_message or 'START' in raw_message or 'END' in raw_message or 'REPORT' in raw_message:
            continue
        try:
            rs.append(json.loads(raw_message))
        except Exception:
            rs.append(raw_message)
    return rs