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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)

...

for i in range(3)
        }
        event = {
            'table_name': table_name,
            'region_name': config.DEFAULT_REGION,
            'items': [{'id': k, 'data': v} for k, v in data.items()]
        }
        self.lambda_client.invoke(
            FunctionName=function_name,
            Payload=json.dumps(event)
        )
        dynamodb = aws_stack.connect_to_resource('dynamodb')
        rs = dynamodb.Table(table_name).scan()
        items = rs['Items']
        self.assertEqual(len(items), len(data.keys()))
        for item in items:
            self.assertEqual(data[item['id']], item['data'])
        
        testutil.delete_lambda_function(function_name)
        dynamodb_client = aws_stack.connect_to_service('dynamodb')
        dynamodb_client.delete_table(TableName=table_name)
    def test_lambda_start_stepfunctions_execution(self):
        function_name = '{}-{}'.format(TEST_LAMBDA_FUNCTION_PREFIX, short_uid())
        resource_lambda_name = '{}-{}'.format(TEST_LAMBDA_FUNCTION_PREFIX, short_uid())
        state_machine_name = 'state-machine-{}'.format(short_uid())
        testutil.create_lambda_function(handler_file=TEST_LAMBDA_START_EXECUTION_FILE,
                                        func_name=function_name,
                                        runtime=LAMBDA_RUNTIME_PYTHON36)
        testutil.create_lambda_function(handler_file=TEST_LAMBDA_ECHO_FILE,
                                        func_name=resource_lambda_name,