Editing users should permit using existing email address
Fix check, because `json_ensure_response` doesn't handle `WP_Error`
Throw an error when a user tries to update to an existing user's email
Simplify check to avoid error in MS
Truly allow user to edit users on MS
Merge pull request #705 from WP-API/fix-608

Throw an error when a user tries to update to an existing user's email