title: Django admin exclude in UserAdmin
body: <p>Currently i have this user proxy model:</p>
  
  <pre><code>class UserProxy(User):
  
      class Meta:
          verbose_name = 'Staff'
          verbose_name_plural = 'Staffs'
          proxy = True
  </code></pre>
  
  <p>And on the admin side i have the following admin like so:</p>
  
  <pre><code>class StaffAdmin(UserAdmin):
      def get_queryset(self, request):
          qs = super(StaffAdmin, self).get_queryset(request)
          return qs.filter(is_staff=True)
  
      exclude = ('first_name', 'last_name',)
  
      def save_model(self, request, obj, form, change):
          if request.user.is_superuser:
              obj.is_staff = True
              obj.save()
  admin.site.register(UserProxy, StaffAdmin)
  </code></pre>
  
  <p>When i go to any form of the proxy model on admin it return the following error:</p>
  
  <blockquote>
    <p>"Key 'first_name' not found in 'UserProxyForm'. Choices are:
    date_joined, email, groups, is_active, is_staff, is_superuser,
    last_login, password, user_permissions, username."</p>
  </blockquote>
  
  <p>I figured that was weird and i tried to only exclude <code>is_staff</code> and now it return:</p>
  
  <blockquote>
    <p>"Key 'is_staff' not found in 'UserProxyForm'. Choices are:
    date_joined, email, first_name, groups, is_active, is_superuser,
    last_login, last_name, password, user_permissions, username."</p>
  </blockquote>
  
  <p>Why is this happening ? isn't the proxy model should have all the fields from base model ? </p>