Fix webdav session login user comparison

On WebDAV sessions (as supported by our client) the provided loginname was compared to the username. They  don't match for LDAP users, for example. It's bad because re-login needs to be done over and over, but functionality itself is not restricted. Needs to be part of next/first maintenance release for OC 6.

Please review @icewind1991 @bartv2 @bantu 

cc @danimo