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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.

self._test_api_gateway_lambda_proxy_integration(
            self.TEST_LAMBDA_PROXY_BACKEND,
            self.API_PATH_LAMBDA_PROXY_BACKEND)
    def test_api_gateway_lambda_proxy_integration_with_path_param(self):
        self._test_api_gateway_lambda_proxy_integration(
            self.TEST_LAMBDA_PROXY_BACKEND_WITH_PATH_PARAM,
            self.API_PATH_LAMBDA_PROXY_BACKEND_WITH_PATH_PARAM)
    def _test_api_gateway_lambda_proxy_integration(self, fn_name, path):
        self.create_lambda_function(fn_name)
        
        lambda_uri = aws_stack.lambda_function_arn(fn_name)
        invocation_uri = 'arn:aws:apigateway:%s:lambda:path/2015-03-31/functions/%s/invocations'
        target_uri = invocation_uri % (aws_stack.get_region(), lambda_uri)
        result = testutil.connect_api_gateway_to_http_with_lambda_proxy(
            'test_gateway2', target_uri, path=path, stage_name=self.TEST_STAGE_NAME)
        api_id = result['id']
        path_map = get_rest_api_paths(api_id)
        _, resource = get_resource_for_path('/lambda/foo1', path_map)
        
        path = path.replace('{test_param1}', 'foo1')
        path = path + '?foo=foo&bar=bar&bar=baz'
        url = gateway_request_url(api_id=api_id, stage_name=self.TEST_STAGE_NAME, path=path)
        data = {'return_status_code': 203, 'return_headers': {'foo': 'bar123'}}
        result = requests.post(url, data=json.dumps(data),
            headers={'User-Agent': 'python-requests/testing'})
        self.assertEqual(result.status_code, 203)
        self.assertEqual(result.headers.get('foo'), 'bar123')
        self.assertIn('set-cookie', result.headers)
        parsed_body = json.loads(to_str(result.content))
        self.assertEqual(parsed_body.get('return_status_code'), 203)