[Push] Fix BUG: RSU cannot push files to Rygel

Rework HTTP requests handling
- Add HTTP HEAD requests support.
- Improve the "getcontentFeatures.dlna.org” request header handling in accordance
with the DLNA specifications.

if “getcontentFeatures.dlna.org” header is present in HTTP GET/HEAD requests, then
if its value is correct (i.e: "1") then we add the “contentFeatures.dlna.org” header
in the HTTP response, else if its value is invalid we return a HTTP "BAD request" error.

if “getcontentFeatures.dlna.org” header is not present in HTTP GET/HEAD requests,
“contentFeatures.dlna.org” header won't be set in the HTTP responses.

Tested with Rygel and XBMC as renderers.

https://github.com/01org/renderer-service-upnp/issues/81

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>