freeze break request for a libsoup bug

[Dan Winship <danw gnome org>]

https://bugzilla.gnome.org/show_bug.cgi?id=646201

Basically, the new added-at-the-last-minute GSettings-based replacement
for the old GConf-based proxy lookup code doesn't correctly deal with
the case of being run in a non-default GMainContext.

In particular, this breaks gstreamer stuff in Epiphany (eg HTML5
audio/video). (Even when no proxy is configured in the control center,
because it still has to make the proxy resolver call to find that out.)

The patch is pretty simple and seems clearly correct to me. (eg, compare
soup_address_resolve_async(), which does exactly the same thing around
its calls to async gio methods).

-- Dan