authform.py: correctly resolve relative path in form action

mandaye/auth/authform.py

@@ -201,7 +201,11 @@ a password_field key if you want to encode a password.")
             action = auth_form.action
 
         if not "://" in action:
-            login_url = re.sub(r'\?.*$', '', self.login_url)
+            basename = os.path.basename(self.login_url)
+            if basename:
+                login_url = self.login_url.split(basename)[0]
+            else:
+                login_url = self.login_url
             action = os.path.join(login_url, action)
 
         cookies = login.cookies