Fix title handling in User::getCanonicalName

Fix title handling in User::getCanonicalName

With r41328 the title validation was changed to allow rename of invalid
user names, by always creating a title object.

That removes the title validation for validation of 'usable' and
'creatable', but in that case it is also a good idea to apply title
rules to normalize spaces and reject user names which can not have a
valid title. At the moment that is done in User::isValidUserName.

Change-Id: I27c488e4008a924117ff6bac2216f148cd5e7abd