This removes workarounds for below libsoup bugs and makes trust-prompt
for WebDAV based backends work better, by checking also server certificates.

 * soup_message_get_https_status() now returns the certificate
   and flags for unsuccessful https connections as well as
   successful ones. [bug #690176]

 * Fixed a deadlock when calling soup_session_abort() on
   a SoupSessionSync in some cases. [bug #691399]