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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.

api_id, stage, relative_path_w_query_params = get_api_id_stage_invocation_path(path, headers)
    relative_path, query_string_params = extract_query_string_params(path=relative_path_w_query_params)
    source_ip = headers.get('X-Forwarded-For', ',').split(',')[-2].strip()
    integration_uri = integration_uri or ''
    account_id = integration_uri.split(':lambda:path')[-1].split(':function:')[0].split(':')[-1]
    domain_name = f'{api_id}.execute-api.{LOCALHOST_HOSTNAME}'
    request_context = {
        
        
        'path': '/' + stage + relative_path,
        'resourcePath': resource_path or relative_path,
        'apiId': api_id,
        'domainPrefix': api_id,
        'domainName': domain_name,
        'accountId': account_id,
        'resourceId': resource_id,
        'stage': stage,
        'identity': {
            'accountId': account_id,
            'sourceIp': source_ip,
            'userAgent': headers.get('User-Agent'),
        },
        'httpMethod': method,
        'protocol': 'HTTP/1.1',
        'requestTime': datetime.datetime.utcnow(),
        'requestTimeEpoch': int(time.time() * 1000),
    }
    return request_context
UPDATE_APIGATEWAY = ProxyListenerApiGateway()