title: email as username, case-sensitive email address
body: <p>I use email address as a username in my Django application</p>
  
  <p><code>USERNAME_FIELD = 'email'</code> but email field is case-sensitive so:</p>
  
  <p><code>test@example.com</code> and <code>TEST@EXAMPLE.COM</code> are saved as two different users. It's normal or should I validate this somehow?</p>