soup-message-queue: hold a reference to the async context
soup-message-queue: hold a reference to the async context

This will ensure that the GMainContext is not freed at least until we get
rid of the queue item. This was causing crashes when synchronously
retrieving resources in WebKit as it uses a different GMainContext for each
of those synchronous requests.

https://bugzilla.gnome.org/show_bug.cgi?id=694920

Distrotech/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,fanc999/libsoup,Distrotech/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,Distrotech/libsoup,Distrotech/libsoup,fanc999/libsoup,fanc999/libsoup,Distrotech/libsoup