Assigning roles is case sensitive, and it shouldn't be

Steps to reproduce:

1. Create a user in Jenkins own user database with upper-case letters.
2. Assign a role to this user, but use lower-case letters.
3. Note that going back into Assign Roles shows the icon indicating that this user is valid.
4. Note that this user does not have the appropriate role.
5. Remove the previous role and assign the role to the user using upper-case letters.
6. Note that the user now has the appropriate role.

Since a user can login to their account in a case insensitive way, the role assignments should also be handled in a case insensitive way. Right now, it is very hard to figure out why roles are not working when the case is not matched between the role screen and the user database.