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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.

else:
                        response._content = to_bytes(parsed_result['body'])
                except Exception:
                    response._content = '{}'
                update_content_length(response)
                response.multi_value_headers = parsed_result.get('multiValueHeaders') or {}
            
            response._content = apply_template(integration, 'response', response._content)
            response.headers['Content-Length'] = str(len(response.content or ''))
            return response
        raise Exception('API Gateway %s integration action "%s", method "%s" not yet implemented' % (
            integration_type, uri, method))
    elif integration_type == 'AWS':
        if 'kinesis:action/' in uri:
            if uri.endswith('kinesis:action/PutRecord'):
                target = kinesis_listener.ACTION_PUT_RECORD
            if uri.endswith('kinesis:action/PutRecords'):
                target = kinesis_listener.ACTION_PUT_RECORDS
            if uri.endswith('kinesis:action/ListStreams'):
                target = kinesis_listener.ACTION_LIST_STREAMS
            template = integration['requestTemplates'][APPLICATION_JSON]
            new_request = aws_stack.render_velocity_template(template, data)
            
            headers = aws_stack.mock_aws_request_headers(service='kinesis')
            headers['X-Amz-Target'] = target
            result = common.make_http_request(url=TEST_KINESIS_URL,
                method='POST', data=new_request, headers=headers)
            
            return result
        elif 'states:action/' in uri: