WL-327 When logging in as an Sakai user keep URL.

When login in as a Sakai user the URL you got redirect to got lost and you got taken back to the Gateway site.
SAK-25884 Don't fail when HELPER_DONE_URL isn't set.

If a login ends up without a HELPER_DONE_URL don't blow up with a stack trace.

git-svn-id: https://source.sakaiproject.org/svn/login/trunk@307230 66ffb92e-73f9-0310-93c1-f5514f145a0a
Merge pull request #3 from ox-it/WL-327

WL-327 Keep URL through login with internal users