Preserve user attributes from providers during authentication

Followup from the PR for

Comment thread starting here:

https://bitbucket.org/opencast-community/matterhorn/pull-requests/1388/mh-12083-fix-possible-npe-when-calling/diff#comment-33241452

If you authenticate via LTI with a username that also exists in Opencast (internally or through a userprovider), the user's provider names, name and email are resolved internally but discarded, as seen in results from /info/me.json:

"user": {
"provider": null,
"name": null,
"email": null,
"username": "01404877"
}

These should be preserved.