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

Now if RSU receives an HTTP HEAD command with the "getContentFeatures.dlna.org"
DLNA header, it will responds with the contentFeatures.dlna.org string instead of
returning "not implemented".

https://github.com/01org/renderer-service-upnp/issues/81

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>