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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.

resource_path, context={}, resource_id=None, response_templates={}):
    response = None
    try:
        response = invoke_rest_api_integration_backend(
            api_id, stage, integration, method, path, invocation_path,
            data, headers, resource_path, context=context, resource_id=resource_id,
            response_templates=response_templates)
        response = apply_response_parameters(response, integration, api_id=api_id)
        return response
    except Exception as e:
        LOG.warning(str(e))
        return make_error_response(str(e), 400)
def invoke_rest_api_integration_backend(api_id, stage, integration, method, path, invocation_path,
        data, headers, resource_path, context={}, resource_id=None, response_templates={}):
    relative_path, query_string_params = extract_query_string_params(path=invocation_path)
    integration_type_orig = integration.get('type') or integration.get('integrationType') or ''
    integration_type = integration_type_orig.upper()
    uri = integration.get('uri') or integration.get('integrationUri') or ''
    try:
        path_params = extract_path_params(path=relative_path, extracted_path=resource_path)
    except Exception:
        path_params = {}
    if (uri.startswith('arn:aws:apigateway:') and ':lambda:path' in uri) or uri.startswith('arn:aws:lambda'):
        if integration_type in ['AWS', 'AWS_PROXY']:
            func_arn = uri
            if ':lambda:path' in uri:
                func_arn = uri.split(':lambda:path')[1].split('functions/')[1].split('/invocations')[0]
            
            data_str = data
            try: