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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.

self.assertDictEqual(update_expected, update_result)
        apig.delete_authorizer(
            restApiId=self.TEST_API_GATEWAY_ID,
            authorizerId=authorizer_id)
        self.assertRaises(
            Exception,
            apig.get_authorizer,
            self.TEST_API_GATEWAY_ID,
            authorizer_id
        )
    def test_apigateway_with_lambda_integration(self):
        apigw
        # creat
        lambda_name = 'apigw-lambda-%s' % short_uid()
        self.create_lambda_function(lambda_name)
        lambda_uri = aws_stack.lambda_function_arn(lambda_name)
        target_uri = aws_stack.apigateway_invocations_arn(lambda_uri)
        # creat
        api = apigw_client.create_rest_api(name='test-api', description='')
        api_id = api['id']
        root_res_id = apigw_client.get_resources(restApiId=api_id)['items'][0]['id']
        api_resource = apigw_client.create_resource(restApiId=api_id, parentId=root_res_id, pathPart='test')
        apigw_client.put_method(
            restApiId=api_id,
            resourceId=api_resource['id'],
            httpMethod='GET',
            authorizationType='NONE'
        )
        rs = apigw_client.put_integration(
            restApiId=api_id,