title: FieldError on django admin fieldsets
body: <p>I try Django from now on several months, I am faced with a strange behavior. An admin model class don't work. But however fields names seems good and the code work properly without <em>fieldsets</em> variable.</p>
  
  <p>Here is the model class :</p>
  
  <pre><code>class Lunch_menu(models.Model):
     lunch_date = models.DateField(auto_now=True)
  
     midday_entrance = models.CharField(max_length=256)
     midday_dish = models.CharField(max_length=256)
     midday_dessert = models.CharField(max_length=256)
  
     dinner_entrance = models.CharField(max_length=256)
     dinner_dish = models.CharField(max_length=256)
     dinner_dessert = models.CharField(max_length=256)
  </code></pre>
  
  <p>The ModelAdmin :</p>
  
  <pre><code>from django.contrib import admin
  from self_menu.models import Lunch_menu
  
  class LunchAdmin(admin.ModelAdmin):
     list_display = ('lunch_date', 'midday_dish', 'dinner_dish')
     fieldsets = (
         (None, {
             'fields': ('lunch_date',)
         }),
         ('Lunch', {
             'classes': ('collapse'),
             'fields': ('midday_entrance', 'midday_dish', 'midday_dessert'),
         }),
     )
  
  admin.site.register(Lunch_menu, LunchAdmin)
  </code></pre>
  
  <p>And finally the error code :</p>
  
  <pre><code>Internal Server Error: /admin/self_menu/lunch_menu/2/change/
  Traceback (most recent call last):
    File "/usr/lib/python3.5/site-packages/django/contrib/admin/options.py", line 639, in get_form
      return modelform_factory(self.model, **defaults)
    File "/usr/lib/python3.5/site-packages/django/forms/models.py", line 545, in modelform_factory
      return type(form)(class_name, (form,), form_class_attrs)
    File "/usr/lib/python3.5/site-packages/django/forms/models.py", line 257, in __new__
      raise FieldError(message)
  django.core.exceptions.FieldError: Unknown field(s) (lunch_date) specified for Lunch_menu
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/usr/lib/python3.5/site-packages/django/core/handlers/base.py", line 149, in get_response
      response = self.process_exception_by_middleware(e, request)
    File "/usr/lib/python3.5/site-packages/django/core/handlers/base.py", line 147, in get_response
      response = wrapped_callback(request, *callback_args, **callback_kwargs)
    File "/usr/lib/python3.5/site-packages/django/contrib/admin/options.py", line 541, in wrapper
      return self.admin_site.admin_view(view)(*args, **kwargs)
    File "/usr/lib/python3.5/site-packages/django/utils/decorators.py", line 149, in _wrapped_view
      response = view_func(request, *args, **kwargs)
    File "/usr/lib/python3.5/site-packages/django/views/decorators/cache.py", line 57, in _wrapped_view_func
      response = view_func(request, *args, **kwargs)
    File "/usr/lib/python3.5/site-packages/django/contrib/admin/sites.py", line 244, in inner
      return view(request, *args, **kwargs)
    File "/usr/lib/python3.5/site-packages/django/contrib/admin/options.py", line 1440, in change_view
      return self.changeform_view(request, object_id, form_url, extra_context)
    File "/usr/lib/python3.5/site-packages/django/utils/decorators.py", line 67, in _wrapper
      return bound_func(*args, **kwargs)
    File "/usr/lib/python3.5/site-packages/django/utils/decorators.py", line 149, in _wrapped_view
      response = view_func(request, *args, **kwargs)
    File "/usr/lib/python3.5/site-packages/django/utils/decorators.py", line 63, in bound_func
      return func.__get__(self, type(self))(*args2, **kwargs2)
    File "/usr/lib/python3.5/contextlib.py", line 30, in inner
      return func(*args, **kwds)
    File "/usr/lib/python3.5/site-packages/django/contrib/admin/options.py", line 1367, in changeform_view
      ModelForm = self.get_form(request, obj)
    File "/usr/lib/python3.5/site-packages/django/contrib/admin/options.py", line 642, in get_form
      % (e, self.__class__.__name__))
  django.core.exceptions.FieldError: Unknown field(s) (lunch_date) specified for Lunch_menu. Check fields/fieldsets/exclude attributes of class LunchAdmin.
  </code></pre>