goawebdavprovider: do not use services_find_preferred() for webdav
Avoid using services_find_preferred()
as WebDAV can not be tested
the same way as CalDAV/CardDAV, with DNS, and WebDAV is not even
listed in services
. Use the provided WebDAV URI or the main URI
as a fallback.
Possibly related to #371 (closed) because the same result occurred to me with a different (but close) WebDAV setup.