Fixed possible request hang with filter finalization.

With r->filter_finalize set the ngx_http_finalize_connection() wasn't
called from ngx_http_finalize_request() called with NGX_OK, resulting in
r->main->count not being decremented, thus causing request hang in some
rare situations.

See here for more details:
http://mailman.nginx.org/pipermail/nginx-devel/2012-May/002190.html

Patch by Yichun Zhang (agentzh).