Fix content filter in ErrorDocument

The content filter in ErrorDocument never set the $done variable to a
true value in the non-subrequest case. As a result, it kept returning
empty strings at the end of input, not undef like it should. This is
actually harmless because response_cb copes with this behavior.
Merge pull request #474 from nwellnhof/error-document-fix

Fix content filter in ErrorDocument