Update the user role hook to include old roles.

[awellis13]

While developing a plugin I needed to see the transition in roles. This is because I am delaying the sending of the new user welcome email until a certain role has been set. So I want to see if a certain role existed before when setting the new role. If yes, send the email.

Of course, I could hook on earlier in the actions and cache the current user's role, but it would be much easier if the old roles were passed into "set_user_role".

Attached is a diff of the change.

[nacin]

Reminded me of the previous change here: #14708.

Looks good.

[SergeyBiryukov]

In 23358:

Pass old roles to set_user_role action. props awellis13. fixes #19134.