librepo currently doesn't allow its clients to set the user agent. this is desirable as the package management clients can then easily identify themselves to the mirror admins who have a better overview of who is using their resources.
Added: https://github.com/Tojaj/librepo/commit/7816dc56f746caebc69294034813fa903bba6681 Fixed in librepo-0.0.2-3