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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)

...

from elasticsearch import Elasticsearch, RequestsHttpConnection
    from requests_aws4auth import AWS4Auth
    env = get_environment(env, region_name=region_name)
    verify_certs = False
    use_ssl = False
    if not endpoint and is_local_env(env):
        endpoint = os.environ['TEST_ELASTICSEARCH_URL']
    if not endpoint and not is_local_env(env) and domain:
        endpoint = get_elasticsearch_endpoint(domain=domain, region_name=env.region)
    
    if 'https://' in endpoint:
        use_ssl = True
        if not is_local_env(env):
            verify_certs = True
    if CUSTOM_BOTO3_SESSION or (ENV_ACCESS_KEY in os.environ and ENV_SECRET_KEY in os.environ):
        access_key = os.environ.get(ENV_ACCESS_KEY)
        secret_key = os.environ.get(ENV_SECRET_KEY)
        session_token = os.environ.get(ENV_SESSION_TOKEN)
        if CUSTOM_BOTO3_SESSION:
            credentials = CUSTOM_BOTO3_SESSION.get_credentials()
            access_key = credentials.access_key
            secret_key = credentials.secret_key
            session_token = credentials.token
        awsauth = AWS4Auth(access_key, secret_key, env.region, 'es', session_token=session_token)
        connection_class = RequestsHttpConnection
        return Elasticsearch(hosts=[endpoint], verify_certs=verify_certs, use_ssl=use_ssl,
                             connection_class=connection_class, http_auth=awsauth)
    return Elasticsearch(hosts=[endpoint], verify_certs=verify_certs, use_ssl=use_ssl)
def create_kinesis_stream(stream_name, shards=1, env=None, delete=False):