<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

# 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.

GATEWAY_RESPONSES[api_id] = GATEWAY_RESPONSES.get(api_id, [])
    data['responseType'] = response_type
    GATEWAY_RESPONSES[api_id].append(data)
    return data
def run_authorizer(api_id, headers, authorizer):
    
    pass
def authorize_invocation(api_id, headers):
    client = aws_stack.connect_to_service('apigateway')
    authorizers = client.get_authorizers(restApiId=api_id, limit=100).get('items', [])
    for authorizer in authorizers:
        run_authorizer(api_id, headers, authorizer)
def validate_api_key(api_key, stage):
    usage_plan_ids = []
    client = aws_stack.connect_to_service('apigateway')
    usage_plans = client.get_usage_plans()
    for item in usage_plans.get('items', []):
        api_stages = item.get('apiStages', [])
        for api_stage in api_stages:
            if api_stage.get('stage') == stage:
                usage_plan_ids.append(item.get('id'))
    for usage_plan_id in usage_plan_ids:
        usage_plan_keys = client.get_usage_plan_keys(usagePlanId=usage_plan_id)
        for key in usage_plan_keys.get('items', []):
            if key.get('value') == api_key:
                return True
    return False
def is_api_key_valid(is_api_key_required, headers, stage):