Remove unnecessary prefix from websocket handler name (#2021)

Remove the websocket prefix "websocket_handler_" introduced in
761eef7. Add a backward support for url_for() calling with this prefix
in param "view_name".

tests/test_url_building.py

@@ -348,3 +348,13 @@ def test_methodview_naming(methodview_app):
 
     assert viewone_url == "/view_one"
     assert viewtwo_url == "/view_two"
+
+
+def test_url_for_with_websocket_handlers(app):
+    # Test for a specific bugfix in GH-2021
+    @app.websocket("/ws")
+    async def my_handler(request, ws):
+        pass
+
+    assert app.url_for("my_handler") == "/ws"
+    assert app.url_for("websocket_handler_my_handler") == "/ws"