Allow changing only the case of user names

Currently, if you try to change only the case of a username (for instance,
from admin to Admin), you'll receive the error message "The username '%s'
already exists".
With this edit we'll let changing the case.

As a further minor improvement, the check if the username is unique is
done only if the username changes (before it was always performed).
Merge pull request #1435 from mlocati/allow-update-usernale-caseonly

Allow changing only the case of user names