Fix appending to IMAP[S] mailboxes

* libproto/imap/appmsg.c (mu_imap_append_message): Fix conditional.
* libproto/imap/mbox.c (_imap_mbx_append_message): Fix error checking.
* libproto/imap/url.c (_mu_imap_url_init)
(_mu_imaps_url_init): Set _get_path member.  This allows the user
to specify an URL without explicit path, implying INBOX.