Improve the omniauth controller specs. Fix the email provided by CAS. Get name from CAS attributes.

Our main issue was the email ending in "@" if no cas_domainname was configured. The specs got improved along the way.
/cc @eriko

* Make omniauth controller specs more robust by using shared examples for all authentication providers in controller spec. – Still passing. Yay!

* Return "casuser", instead of "casuser@" when no cas_domainname is configured.

* If no cas_domainname is configured, the CAS authentication would return "casuser@" for the users email field, because it tried to assume the email adress of the CAS user by it's username + cas_domainname.
  Now it just returns the username instead of adding an "@" if cas_domainname is not configured.
  This especially makes sense on CAS setups where the username equals the users email adress.
  The old behaviour, if cas_domainname is configured, was not changed.

* Fetch the email from CAS attributes if provided
  If the cas:authenticationSuccess (handled via omniauth-cas) response gives us an email use that.
  If not, behave as before (username or username@cas_domainname).

* Fetch the (full) name from CAS attributes if provided
  If the CAS response by omniauth provides a [:info][:name] field, prefer this over the uid, because we want the name to be a "Full Name", instead of just a "shortname"