Fixing bug repoted in issue #1324: (#1325)

- Added proper call to has_request_context function

Co-authored-by: igor_kantorski <igor.kantorski@globedata.pl>

flask_socketio/__init__.py

@@ -408,7 +408,7 @@ def ping():
         if callback:
             # wrap the callback so that it sets app app and request contexts
             sid = None
-            if has_request_context:
+            if has_request_context():
                 sid = getattr(flask.request, 'sid', None)
             original_callback = callback