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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']
        # test 
        try:
            client.get_(restApiId=rest_api_id, Name='fake')
            self.fail('This call should not be successful as the  is not created.')
        except ClientError as e:
            self.assertEqual(e.response['Error']['Code'], 'NotFoundException')
        
        client.delete_rest_api(restApiId=rest_api_id)
    def test_put_integration_dynamodb_proxy_validation_without_response_template(self):
        api_id = self.create_api_gateway_and_deploy({})
        url = gateway_request_url(api_id=api_id, stage_name='staging', path='/')
        response = requests.put(
            url,
            json.dumps({'id': 'id1', 'data': 'foobar123'}),
        )
        self.assertEqual(response.status_code, 404)
    def test_put_integration_dynamodb_proxy_validation_with_response_template(self):