Fix for incorrect 201 replies from dav module.

Fix for incorrect 201 replies from dav module.

Replies with 201 code contain body, and we should clearly indicate it's
empty if it's empty.  Before 0.8.32 chunked was explicitly disabled for
201 replies and as a result empty body was indicated by connection close
(not perfect, but worked).  Since 0.8.32 chunked is enabled, and this
causes incorrect responses from dav module when HTTP/1.1 is used: with
"Transfer-Encoding: chunked" but no chunks at all.

Fix is to actually return empty body in special response handler instead
of abusing r->header_only flag.

See here for initial report:
http://mailman.nginx.org/pipermail/nginx-ru/2010-October/037535.html