title: undefined method will_save_change_to_email?
body: <p>I started an application with Rails 5.1.4 ; Devise 4.2 ; Mongoid 6.1.0.</p>
  
  <p>When I trying to create an account, I got this error</p>
  
  <blockquote>
    <p>NoMethodError in Devise::RegistrationsController#create undefined
    method `will_save_change_to_email?' for #</p>
  </blockquote>
  
  <p>I see some post on GitHub but I don't know how to fix that.
  <a href="https://github.com/plataformatec/devise/issues/4531#issuecomment-313600769" rel="nofollow noreferrer">https://github.com/plataformatec/devise/issues/4531#issuecomment-313600769</a></p>
  
  <p>What should I do?</p>