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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.

'throttle': {'rateLimit': 2, 'burstLimit': 1},
                'apiStages': [{'apiId': api_id, 'stage': 'staging'}],
                'tags': {'tag_key': 'tag_value'},
            }
            usage_plan_ids.append(client.create_usage_plan(**payload)['id'])
        api_keys = []
        key_type = 'API_KEY'
        # Creat
        for usage_plan_id in usage_plan_ids:
            for i in range(2):
                api_key = client.create_api_key(name='testMultipleApiKeys{}'.format(i))
                payload = {'usagePlanId': usage_plan_id, 'keyId': api_key['id'], 'keyType': key_type}
                client.create_usage_plan_key(**payload)
                api_keys.append(api_key['value'])
        response = requests.put(
            url,
            json.dumps({'id': 'id1', 'data': 'foobar123'}),
        )
        # when 
        self.assertEqual(response.status_code, 403)
        # check 
        for key in api_keys:
            response = requests.put(
                url,
                json.dumps({'id': 'id1', 'data': 'foobar123'}),
                headers={'X-API-Key': key}
            )
            # when 
            self.assertEqual(response.status_code, 200)
    def test_import_rest_api(self):