Fix soup_client_input_stream_close to not block

Closing a SoupClientInputStream for a message that hadn't been
completely read was trying to read to the end of the message first.
Fix it to just cancel the read instead.

Also fix a few tests that were implicitly assuming the old behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=695652
tests/connection-test: Ensure quit source id is reset to 0 between runs

Otherwise we may attempt to remove a nonexistent quit loop from the
previous one.  The second hunk here is the real fix, the first is just
being more defensive/consistent.

Reported-By: Kjartan Maraas <kmaraas@gnome.org>
Tested-By: Kjartan Maraas <kmaraas@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=711361
2.42.3