Error 501 is shown when sending invalid data in hidden form

Prerequisites:

• Firefox Developer tool must be installed
• User should have "View topic" permissions
• Topic must be created

Steps to reproduce:

1. Sing In
2. Go to topic page
3. Select "Display hidden elements" in "Miscellaneous" menu on the developer tool panel
4. Enter any text in hidden field that located after "Now are browsing this branch", but before "Powered by JCommune"
5. Press [Ctrl]+[Enter]

AR: An error with HTTP status 501 appears:"HTTP Status 501 - Method ******* is not is not implemented by this servlet for this URI"
ER: User can't send any request in this field.

Note: Can be reproduce on CR page also