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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)

...

lambda_name2 = 'test2-%s' % short_uid()
            testutil.create_lambda_function(func_name=lambda_name2,
                                            zip_file=python3_with_settings2,
                                            runtime=LAMBDA_RUNTIME_PYTHON36,
                                            handler='handler2.handler')
            result1 = self.lambda_client.invoke(FunctionName=lambda_name1, Payload=b'{}')
            result_data1 = result1['Payload'].read()
            result2 = self.lambda_client.invoke(FunctionName=lambda_name2, Payload=b'{}')
            result_data2 = result2['Payload'].read()
            self.assertEqual(result1['StatusCode'], 200)
            self.assertIn('setting1', to_str(result_data1))
            self.assertEqual(result2['StatusCode'], 200)
            self.assertIn('setting2', to_str(result_data2))
            
            testutil.delete_lambda_function(lambda_name1)
            testutil.delete_lambda_function(lambda_name2)
        finally:
            lambda_api.DO_USE_DOCKER = original_do_use_docker
    def test_lambda_subscribe_sns_topic(self):
        function_name = '{}-{}'.format(TEST_LAMBDA_FUNCTION_PREFIX, short_uid())
        testutil.create_lambda_function(handler_file=TEST_LAMBDA_ECHO_FILE,
                                        func_name=function_name, runtime=LAMBDA_RUNTIME_PYTHON36)
        topic = self.sns_client.create_topic(Name=TEST_SNS_TOPIC_NAME)
        topic_arn = topic['TopicArn']
        self.sns_client.subscribe(
            TopicArn=topic_arn,
            Protocol='lambda',
            Endpoint=lambda_api.func_arn(function_name),
        )
        subject = '[Subject] Test subject'