Fixed the email already exist issue for edit user

The issue was reported here https://github.com/WP-API/WP-API/issues/608 by me, and I have added the fix here
@danielbachhuber yes we can prefix it with `json_`, but this is something which is used in wp core as well as also used by the WP API plugin for "new user" functionality. Please let me know how to go ahead with this