GDBusConnection: Use correct GMainContext when invoking free functions

GDBusConnection: Use correct GMainContext when invoking free functions

Without this fix, the ./gdbus-connection test case occasionally fails, see

 https://bugzilla.gnome.org/show_bug.cgi?id=629945#c5

like this

 /gdbus/connection/basic: OK
 /gdbus/connection/life-cycle: **
ERROR:gdbus-connection.c:223:test_connection_life_cycle: assertion failed:
(!quit_mainloop_fired)
 cleaning up bus with pid 21794
 Aborted (core dumped)

because the callback didn't happen on the same thread as where we are
running the loop.

Signed-off-by: David Zeuthen <davidz@redhat.com>