Fix #1788 incorrect url_for for routes with hosts, added tests. (#1789)

* Fix #1788 incorrect url_for for routes with hosts, added tests.

* Linter

* Remove debug print

tests/test_url_for.py

@@ -0,0 +1,12 @@
+def test_routes_with_host(app):
+    @app.route("/")
+    @app.route("/", name="hostindex", host="example.com")
+    @app.route("/path", name="hostpath", host="path.example.com")
+    def index(request):
+        pass
+
+    assert app.url_for("index") == "/"
+    assert app.url_for("hostindex") == "/"
+    assert app.url_for("hostpath") == "/path"
+    assert app.url_for("hostindex", _external=True) == "http://example.com/"
+    assert app.url_for("hostpath", _external=True) == "http://path.example.com/path"