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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)

...

MAVEN_BASE_URL = 'https://repo.maven.apache.org/maven2'
TEST_LAMBDA_JAR_URL = '{url}/cloud/localstack/{name}/{version}/{name}-{version}-tests.jar'.format(
    version=LOCALSTACK_MAVEN_VERSION, url=MAVEN_BASE_URL, name='localstack-utils')
TEST_LAMBDA_LIBS = [
    'localstack', 'localstack_client', 'requests', 'psutil', 'urllib3', 'chardet', 'certifi', 'idna', 'pip', 'dns'
]
def _run_forward_to_fallback_url(url, num_requests=3):
    lambda_client = aws_stack.connect_to_service('lambda')
    config.LAMBDA_FALLBACK_URL = url
    try:
        for i in range(num_requests):
            lambda_client.invoke(FunctionName='non-existing-lambda-%s' % i,
                                 Payload=b'{}', InvocationType='RequestResponse')
    finally:
        config.LAMBDA_FALLBACK_URL = ''
class LambdaTestBase(unittest.TestCase):
    def test_create_lambda_function(self):
        func_name = 'lambda_func-{}'.format(short_uid())
        kms_key_arn = 'arn:aws:kms:us-east-1:000000000000:key11'
        vpc_config = {
            'SubnetIds': ['subnet-123456789'],
            'SecurityGroupIds': ['sg-123456789']
        }
        tags = {
            'env': 'testing'
        }
        kwargs = {
            'FunctionName': func_name,
            'Runtime': LAMBDA_RUNTIME_PYTHON37,
            'Handler': LAMBDA_DEFAULT_HANDLER,