Do not set role if it is the same

[nickmomrik]

When set_role is called it doesn't check to see if the role is changing. So we can update the user and fire the set_user_role action even when nothing is changing.

To reproduce go to the Users screen and "change" the role of an Administrator to an Administrator. Or use wp-admin/ms-sites.php to update options. The user will be updated and the set_user_role action fires.

Possible fix

​http://lists.automattic.com/pipermail/wp-hackers/2010-July/033194.html

[15919]