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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)

...

'StreamViewType': stream_view_type
        }
    table = None
    try:
        table = dynamodb.create_table(TableName=table_name, KeySchema=key_schema,
            AttributeDefinitions=attr_defs, ProvisionedThroughput={
                'ReadCapacityUnits': 10, 'WriteCapacityUnits': 10
            },
            StreamSpecification=stream_spec
        )
    except Exception as e:
        if 'ResourceInUseException' in str(e):
            
            return connect_to_resource('dynamodb', env=env).Table(table_name)
    time.sleep(2)
    return table
def get_apigateway_integration(api_id, method, path, env=None):
    apigateway = connect_to_service(service_name='apigateway', client=True, env=env)
    resources = apigateway.get_resources(restApiId=api_id, limit=100)
    resource_id = None
    for r in resources['items']:
        if r['path'] == path:
            resource_id = r['id']
    if not resource_id:
        raise Exception('Unable to find apigateway integration for path "%s"' % path)
    integration = apigateway.get_integration(
        restApiId=api_id, resourceId=resource_id, httpMethod=method
    )
    return integration
def get_apigateway_resource_for_path(api_id, path, parent=None, resources=None):