It looks like a duplicate of old issue https://github.com/localstack/localstack/issues/78
But now it's reproducible for me with the latest version of LocalStack. 
def forward_request(self, **kwargs):
                response = Response()
                response.status_code = 200
                result = {
                    'data': kwargs.get('data') or '{}',
                    'headers': dict(kwargs.get('headers'))
                }
                response._content = json.dumps(json_safe(result))
                return response
        proxy = GenericProxy(test_port, update_listener=TestListener())
        proxy.start()
        
        result = self.connect_api_gateway_to_http(
            'test_gateway2',