GUPnP 0.20.4 Makes the User-Agent ASCII-Only

GUPnP 0.20.4 fixes warnings in gtk-doc run, fixes codestyle issues, adds minor improvements to the documentation, and makes the user-agent ASCII-only in order to repair some issues with discovery when the g_get_application_name () API returned a translated string. Moreover, GUPnP 0.20.4 forces the HTTP version to be 1.1 in SOAP requests, fixes potential crashes in XML parsing, fixes a potential memory leak in the gupnp_service_proxy_send_action_valist function, and adds the gupnp_device_info_list_dlna_device_class_identifier () API to retrieve the content of nodes.