Added clearing of r->valid_unparsed_uri on internal redirects.

Added clearing of r->valid_unparsed_uri on internal redirects.

This resolves issue with try_files (see ticket #70), configuration like

   location / { try_files $uri /index.php; }
   location /index.php { proxy_pass http://backend; }

caused nginx to use original request uri in a request to a backend.

Historically, not clearing of the r->valid_unparsed_uri on internal redirect
was a feature: it allowed to pass the same request to (another) upstream
server via error_page redirection.  Since then named locations appeared
though, and it's time to start resetting r->valid_unparsed_uri on internal
redirects.  Configurations still using this feature should be converted
to use named locations instead.

Patch by Lanshun Zhou.