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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)

...

if resources is None:
        apigateway = connect_to_service(service_name='apigateway')
        resources = apigateway.get_resources(restApiId=api_id, limit=100)
    if not isinstance(path, list):
        path = path.split('/')
    if not path:
        return parent
    for resource in resources:
        if resource['pathPart'] == path[0] and (not parent or parent['id'] == resource['parentId']):
            return get_apigateway_resource_for_path(api_id, path[1:], parent=resource, resources=resources)
    return None
def get_apigateway_path_for_resource(api_id, resource_id, path_suffix='', resources=None, region_name=None):
    if resources is None:
        apigateway = connect_to_service(service_name='apigateway', region_name=region_name)
        resources = apigateway.get_resources(restApiId=api_id, limit=100)['items']
    target_resource = list(filter(lambda res: res['id'] == resource_id, resources))[0]
    path_part = target_resource.get('pathPart', '')
    if path_suffix:
        if path_part:
            path_suffix = '%s/%s' % (path_part, path_suffix)
    else:
        path_suffix = path_part
    parent_id = target_resource.get('parentId')
    if not parent_id:
        return '/%s' % path_suffix
    return get_apigateway_path_for_resource(api_id, parent_id,
        path_suffix=path_suffix, resources=resources, region_name=region_name)
def create_api_gateway(name, description=None, resources=None, stage_name=None,