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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.

result = requests.options(url, headers={'origin': origin})
        self.assertEqual(result.status_code, 200)
        self.assertTrue(re.match(result.headers['Access-Control-Allow-Origin'].replace('*', '.*'), origin))
        self.assertIn('POST', result.headers['Access-Control-Allow-Methods'])
        self.assertIn('PATCH', result.headers['Access-Control-Allow-Methods'])
        custom_result = json.dumps({'foo': 'bar'})
        
        result = requests.get(url)
        self.assertEqual(result.status_code, 200)
        expected = custom_result if int_type == 'custom' else '{}'
        self.assertEqual(json.loads(to_str(result.content))['data'], expected)
        
        data = json.dumps({'data': 123})
        result = requests.post(url, data=data)
        self.assertEqual(result.status_code, 200)
        expected = custom_result if int_type == 'custom' else data
        self.assertEqual(json.loads(to_str(result.content))['data'], expected)
        
        data = 'test=123'
        ctype = 'application/x-www-form-urlencoded'
        result = requests.post(url, data=data, headers={'content-type': ctype})
        self.assertEqual(result.status_code, 200)
        content = json.loads(to_str(result.content))
        headers = CaseInsensitiveDict(content['headers'])
        expected = custom_result if int_type == 'custom' else data
        self.assertEqual(content['data'], expected)
        self.assertEqual(headers['content-type'], ctype)
        
        proxy.stop()
    def test_api_gateway_lambda_proxy_integration(self):