GSettings: fix check for delaying backend subscription

g_settings_has_signal_handlers() checks whether any of the signals has
pending handlers. However, g_signal_has_handler_pending() matches on
exact detail, even when passing 0. Subscribing to one of GSettings'
signals with a detail will fail this check and never connect to the
backend.

Fix this by calling has_handler_pending() with the key as detail as
well.

https://bugzilla.gnome.org/show_bug.cgi?id=740848