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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.

combined = {}
                combined.update(path_params or {})
                combined.update(query_params or {})
                combined.update(headers or {})
                return combined if not name else combined.get(name)
            context['params'] = _params
            data = aws_stack.render_velocity_template(template, context)
    return data
def apply_response_parameters(response, integration, api_id=None):
    int_responses = integration.get('integrationResponses') or {}
    if not int_responses:
        return response
    entries = list(int_responses.keys())
    return_code = str(response.status_code)
    if return_code not in entries:
        if len(entries) > 1:
            LOG.info('Found multiple integration response status codes: %s' % entries)
            return response
        return_code = entries[0]
    response_params = int_responses[return_code].get('responseParameters', {})
    for key, value in response_params.items():
        
        if str(key).lower().startswith('method.response.header.'):
            header_name = key[len('method.response.header.'):]
            response.headers[header_name] = value.strip("'")
    return response
def get_api_id_stage_invocation_path(path, headers):