Merge pull request tornadoweb#1281 from nordaux/master

Fixed error that the web.asynchronous doesn't complete python3 concurrent.Futures

tornado/web.py

@@ -1478,7 +1478,7 @@ def wrapper(self, *args, **kwargs):
         with stack_context.ExceptionStackContext(
                 self._stack_context_handle_exception):
             result = method(self, *args, **kwargs)
-            if isinstance(result, Future):
+            if is_future(result):
                 # If @asynchronous is used with @gen.coroutine, (but
                 # not @gen.engine), we can automatically finish the
                 # request when the future resolves.  Additionally,