It's unused and features are already attached to a session with
SoupSessionFeature::attach.

SoupSession now notifies all the features about message being queued and
unqueued. This way features implementing attach don't need to chain up,
since there's no longer a default implementation for attach and detach.

Features can be removed at any point from a session, and if they are
removed while there are messages in the queue, it is possible for
callbacks connected to the message signals to be called after the feature
has been disposed. Adding a reference in request_queue()
and removing it in request_enqueue() ensures that the feature will not
be disposed too early.

Add a test case for the SoupCookieJar feature that reproduces this crash.

Fixes #130

The *_default_init() function is declared by G_DEFINE_INTERFACE, so
doesn’t have to be declared separately. This eliminates some compiler
warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=748514

It's similar to the SoupSession::request-started signal, but it's also
emitted for cached resources.

https://bugzilla.gnome.org/show_bug.cgi?id=731153

Add g_return_if_fail() checks to soup_session_feature_attach() and
soup_session_feature_detach(), to work around a bug in Eclipse that
was fixed a long time ago
(https://bugs.eclipse.org/bugs/show_bug.cgi?id=404776) but which
people keep reporting against libsoup.

https://bugzilla.gnome.org/show_bug.cgi?id=710242