title: Using 'django-filter' with CHOICES field - need "Any" option
body: <p>I'm using the very cool django-filter (via: <a href="http://github.com/alex/django-filter" rel="noreferrer">http://github.com/alex/django-filter</a>)
  and either can't seem to wrap my head around the docs, or maybe just
  need a little boost.</p>
  
  <p>When I show the filter form on an object list page, for a FK field I
  get the drop down that includes a "-----" which results in an
  "any" type filter. But I have some choices set to a field on that
  model, and I'd like to get the same "any" type option.
  Here's a relevant example portion from models.py:</p>
  
  <pre><code>TICKET_STATUS_CHOICES = (
      ('new', 'New'),
      ('accepted', 'Accepted'),
      ('assigned', 'Assigned'),
      ('reopened', 'Reopened'),
      ('closed', 'Closed'),
  )
  
  class Ticket(models.Model):
      assigned_to = models.ForeignKey(User, null=True, blank=True)
      status = models.CharField(max_length=20,
  choices=TICKET_STATUS_CHOICES, default='new')
  
  import django_filters
  
  class TicketFilter(django_filters.FilterSet):
      class Meta:
          model = Ticket
          fields = ['assigned_to', 'status']
  </code></pre>
  
  <p>When I display the filter form, '<code>assigned_to'</code> gets an 'any' option, as
  well as listing the available users. The <code>'status'</code> field, however, is
  limited to only the options listed in the actual '_CHOICES'.</p>
  
  <p>How do I add an 'any' option to the fields based on _CHOICES? </p>