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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)

...

source = '*' if source_path == base_dir else os.path.basename(source_path)
    command = 'cd %s; zip -r %s %s' % (base_dir, zip_file, source)
    run(command)
def create_zip_file_python(source_path, base_dir, zip_file):
    with zipfile.ZipFile(zip_file, 'w') as zip_file:
        for root, dirs, files in os.walk(base_dir):
            for name in files:
                full_name = os.path.join(root, name)
                relative = root[len(base_dir):].lstrip(os.path.sep)
                dest = os.path.join(relative, name)
                zip_file.write(full_name, dest)
def create_zip_file(file_path, get_content=False):
    base_dir = file_path
    if not os.path.isdir(file_path):
        base_dir = tempfile.mkdtemp(prefix=ARCHIVE_DIR_PREFIX)
        shutil.copy(file_path, base_dir)
        TMP_FILES.append(base_dir)
    tmp_dir = tempfile.mkdtemp(prefix=ARCHIVE_DIR_PREFIX)
    zip_file_name = 'archive.zip'
    full_zip_file = os.path.join(tmp_dir, zip_file_name)
    
    if is_alpine():
        create_zip_file_cli(file_path, base_dir, zip_file=full_zip_file)
    else:
        create_zip_file_python(file_path, base_dir, zip_file=full_zip_file)
    if not get_content: