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# Type of request: This is a ...

[ ] bug report
[x] feature request

# Detailed description

The API Gateway response body only accepts strings. To transfer binary data, it needs to be base64 encoded, inserted as a string in the response body and the flag ´isBase64Encoded´ must be set. Once this is done API Gateway will base64 decode the response and return the binary content to the requester.

## Expected behavior

When setting the isBase64Encoded flag, the response is base64 decoded before being returned from API Gateway.

## Actual behavior

Setting the isBase64Encoded flag has no effect, the response body will contain the base64 encoded string instead of binary data.

# Steps to reproduce

## Command used to start LocalStack

Docker compose config:
```
version: '2.1'

services:
  docker-host:
    image: qoomon/docker-host:2.5.0
    cap_add: [ 'NET_ADMIN', 'NET_RAW' ]
    mem_limit: 20M
    restart: on-failure
  localstack:
    image: localstack/localstack:0.12.10
    depends_on: [ docker-host ]
    ports:
      - "4566:4566"
      - "4571:4571"
    environment:
      - DOCKER_HOST=unix:///var/run/docker.sock
      - HOST_TMP_FOLDER=${TMPDIR}
      - DATA_DIR=${TMPDIR}/data
      - DEFAULT_REGION=eu-west-1
      - LAMBDA_REMOTE_DOCKER=0
      - LAMBDA_EXECUTOR=docker-reuse
      - LAMBDA_DOCKER_NETWORK=scripts_default
      - AWS_CBOR_DISABLE=true
    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)

Need to set up a Lambda Rest API that returns a base64 encoded string, where the flag `isBase64Encoded` is set. The returned response should reflect the base64 decoded version of the string.

client = aws_stack.connect_to_service('apigateway')
        response = client.create_rest_api(name='my_api', description='this is my api')
        rest_api_id = response['id']
        # CREATE
        name = 'validator123'
        result = client.create_request_validator(restApiId=rest_api_id, name=name)
        self.assertEqual(result['ResponseMetadata']['HTTPStatusCode'], 200)
        validator_id = result['id']
        # LIST
        result = client.get_request_validators(restApiId=rest_api_id)
        self.assertEqual(result['ResponseMetadata']['HTTPStatusCode'], 200)
        self.assertEqual(result['items'], [{'id': validator_id, 'name': name}])
        # GET
        result = client.get_request_validator(restApiId=rest_api_id, requestValidatorId=validator_id)
        self.assertEqual(result['ResponseMetadata']['HTTPStatusCode'], 200)
        self.assertEqual(select_attributes(result, ['id', 'name']), {'id': validator_id, 'name': name})
        # UPDATE
        result = client.update_request_validator(restApiId=rest_api_id, requestValidatorId=validator_id,
            patchOperations=[])
        # DELETE
        client.delete_request_validator(restApiId=rest_api_id, requestValidatorId=validator_id)
        with self.assertRaises(Exception):
            client.get_request_validator(restApiId=rest_api_id, requestValidatorId=validator_id)
        with self.assertRaises(Exception):
            client.delete_request_validator(restApiId=rest_api_id, requestValidatorId=validator_id)
        
        client.delete_rest_api(restApiId=rest_api_id)
    def test_base_path_mapping(self):