soup-socket: fix another leak in soup_socket_get_http_proxy_uri()

Add a test for the leak in bug 700472, which also turned up another
leak.

https://bugzilla.gnome.org/show_bug.cgi?id=700472
use libfffi-based generic marshaller instead of glib-genmarshal

g_cclosure_marshal_generic() is the default signal handler starting
from glib 2.29.12. libsoup already requires glib 2.33.1.

https://bugzilla.gnome.org/show_bug.cgi?id=686042
oops, addendum to previous commit