Fix using non-strings as anchors

Previously, non-strings passed to _anchor would be converted to strings.
Restore this ability. The logic is the same as in url_quote.

Fixes: c4b39ba2 ("replace werkzeug.urls with urllib.parse")

tests/test_helpers.py

@@ -106,6 +106,7 @@ class TestUrlFor:
             return "42"
 
         assert flask.url_for("index", _anchor="x y") == "/#x%20y"
+        assert flask.url_for("index", _anchor=5) == "/#5"
 
     def test_url_for_with_scheme(self, app, req_ctx):
         @app.route("/")