use the exact login name for the user display name on registration

As per Brian's suggestion, when creating a zooniverse user account (no omniauth) we take the login name as typed and set to the display name (no form field for display name). This avoids the issue of the login name being downcased for display and we preserve the correct formatting for display.
Can we use this [helper method](https://github.com/zooniverse/Panoptes/blob/master/spec/support/controller_helpers.rb#L10-L12) instead? Just to keep it consistent.