http: Fix parsing DAV:getlastmodified

It's defined as a rfc1123 date which always has a "GMT" suffix (which means
UTC). The Qt::RFC2822Date parser in Qt no longer accepts timezones declared
as obsolete in RFC2822, even though that RFC says it MUST be accepted...

Also add some error handling to avoid spitting out invalid UDS values.

BUG: 484671