Return the actual future from async_send_request (ros#209)

Recent changes to rclcpp in Rolling make it return a structure
rather than just the future.  In order to maintain this test,
return the actual future.

Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>

self_test/src/run_selftest.cpp

@@ -91,7 +91,7 @@ class ClientNode : public rclcpp::Node
           }
         }
       };
-    return client_->async_send_request(request, response_received_callback);
+    return client_->async_send_request(request, response_received_callback).future;
   }
 
 private: