Content type for PROPFIND responses.

We've spotted a shortfall in the PROPFIND responses. They have no Content-Type header set.

>   I've attached a patch for ngx_http_dav_ext_module.c that sets a Content-Type: application/xml
>   header when I believe the output will always be XML. The absence of this 
>   header is what prevents the Perl HTTP::DAV client from working. 
>   AFAICT, with reference to RFC 4918, this header is required. See:
>   http://www.webdav.org/specs/rfc4918.html#rfc.section.8.2