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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.

TEST_STAGE_NAME = 'testing'
    TEST_LAMBDA_PROXY_BACKEND = 'test_lambda_apigw_backend'
    TEST_LAMBDA_PROXY_BACKEND_WITH_PATH_PARAM = 'test_lambda_apigw_backend_path_param'
    TEST_LAMBDA_PROXY_BACKEND_ANY_METHOD = 'test_lambda_apigw_backend_any_method'
    TEST_LAMBDA_PROXY_BACKEND_ANY_METHOD_WITH_PATH_PARAM = 'test_lambda_apigw_backend_any_method_path_param'
    TEST_LAMBDA_SQS_HANDLER_NAME = 'lambda_sqs_handler'
    TEST_LAMBDA_AUTHORIZER_HANDLER_NAME = 'lambda_authorizer_handler'
    TEST_API_GATEWAY_ID = 'fugvjdxtri'
    TEST_API_GATEWAY_AUTHORIZER = {
        'name': 'test',
        'type': 'TOKEN',
        'providerARNs': [
            'arn:aws:cognito-idp:us-east-1:123412341234:userpool/us-east-1_123412341'
        ],
        'authType': 'custom',
        'authorizerUri': 'arn:aws:apigateway:us-east-1:lambda:path/2015-03-31/functions/' +
                         'arn:aws:lambda:us-east-1:123456789012:function:myApiAuthorizer/invocations',
        'authorizerCredentials': 'arn:aws:iam::123456789012:role/apigAwsProxyRole',
        'identitySource': 'method.request.header.Authorization',
        'identityValidationExpression': '.*',
        'authorizerResultTtlInSeconds': 300
    }
    TEST_API_GATEWAY_AUTHORIZER_OPS = [
        {
            'op': 'replace',
            'path': '/name',
            'value': 'test1'
        }
    ]
    def test_api_gateway_kinesis_integration(self):