Filter should execute for 404 dispatched requests

The commit 7b85438 introduced a failure for error pages that should be
processed by the convention.
There wasn't actually a reasoning for that implementation, other than
avoid the compexity of dealing with recursive calls. Besides, it wasn't
backed up by any test.
There is a check to see whether the URL has a valid page or not, so
recursive calls should not be a problem anyway.

Closes gh-11.