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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.

resourceId=api_resource['id'],
            httpMethod='GET',
            integrationHttpMethod='POST',
            type='AWS',
            uri=target_uri,
            timeoutInMillis=3000,
            contentHandling='CONVERT_TO_BINARY',
            requestTemplates={
                'application/json': '{"param1": "$input.params(\'param1\')"}'
            }
        )
        integration_keys = ['httpMethod', 'type', 'passthroughBehavior', 'cacheKeyParameters', 'uri', 'cacheNamespace',
            'timeoutInMillis', 'contentHandling', 'requestParameters']
        self.assertEqual(rs['ResponseMetadata']['HTTPStatusCode'], 200)
        for key in integration_keys:
            self.assertIn(key, rs)
        self.assertNotIn('responseTemplates', rs)
        apigw_client.create_deployment(restApiId=api_id, stageName=self.TEST_STAGE_NAME)
        rs = apigw_client.get_integration(
            restApiId=api_id,
            resourceId=api_resource['id'],
            httpMethod='GET'
        )
        self.assertEqual(rs['ResponseMetadata']['HTTPStatusCode'], 200)
        self.assertEqual(rs['type'], 'AWS')
        self.assertEqual(rs['httpMethod'], 'POST')
        self.assertEqual(rs['uri'], target_uri)
        # invok
        url = gateway_request_url(api_id=api_id, stage_name=self.TEST_STAGE_NAME, path='/test')
        response = requests.get('%s?param1=foobar' % url)