SoupSessionAsync: don't queue idles from dispose()

SoupSessionAsync: don't queue idles from dispose()

SoupSession does a soup_session_abort() from dispose(), which had the
effect in SoupSessionAsync of queueing an idle (to check if new
messages could be sent now that old connections had been closed). This
causes problems if the session was disposed from a thread other than
the one that its GMainContext is running in, which is weird, and also
incompatible with some garbage-collected runtimes. So fix that.

https://bugzilla.gnome.org/show_bug.cgi?id=667364