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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.

if not methods:
            methods = ['GET', 'POST']
        if not path:
            path = '/'
        resources = {}
        resource_path = path.replace('/', '')
        resources[resource_path] = []
        req_templates = {
            'application/json': json.dumps({'foo': 'bar'})
        } if int_type == 'custom' else {}
        for method in methods:
            resources[resource_path].append({
                'httpMethod': method,
                'integrations': [{
                    'type': 'HTTP' if int_type == 'custom' else 'HTTP_PROXY',
                    'uri': target_url,
                    'requestTemplates': req_templates,
                    'responseTemplates': {}
                }]
            })
        return aws_stack.create_api_gateway(
            name=gateway_name,
            resources=resources,
            stage_name=self.TEST_STAGE_NAME
        )
    @staticmethod
    def create_lambda_function(fn_name):
        testutil.create_lambda_function(
            handler_file=TEST_LAMBDA_PYTHON, libs=TEST_LAMBDA_LIBS, func_name=fn_name)
    @staticmethod