`PUT User` shouldn't permit using existing `user_login` or
`user_nicename`
Don't permit `username` to be edited

Core silently discards the value, anyway. Better to be verbose about it
Correct response message
Throw an error if a duplicate slug is provided
Merge pull request #707 from WP-API/user-update-userlogin

`PUT User` shouldn't permit using existing `user_login` or `user_nicename`