title: field does not allow filtering (no relation used)
body: <p>I have a problem with django and tastypie</p>
  
  <p>Given is the following code:</p>
  
  <pre><code>class CandidatePollResource(ModelResource):
      class Meta:
          queryset = Candidate.objects.all()
          resource_name = "candidate-poll"
          filtering = {"status": ALL }
  
  class Candidate(Profile):
      """
      This profile stores all information about a candidate.
      """
      status = models.CharField(_('status'), max_length=15, blank=True, choices=CANDIDATE_STATUS_CHOICES)
  
  class Profile(models.Model):
      """
      Abstract basic class all profiles should inherit.
      """
      user = models.OneToOneField(CustomUser,related_name='%(class)s',)
      invitationdate = models.DateField(_('invitationdate'), null=True, blank=True)
      confirmationdate = models.DateField(_('confirmationdate'), null=True, blank=True)
      activation_key = models.CharField(_('activation_key'), max_length=32, blank=True)
      # Adding of "class" here is important for this to work. See
      # http://thedjangoforum.com/board/thread/417/reverse-query-name-for-field-clash/?page=1
      created_by = models.ForeignKey(CustomUser, related_name='%(class)s_created', blank=True, null=True)
  
      objects = ProfileManager()
  
      class Meta:
          abstract = True
  </code></pre>
  
  <p>Each time we try to do a call to filter the result set ( <code>http://localhost:3000/api/v1/candidate-poll/?status__exact=new</code> ), I always get the following error</p>
  
  <p><code>The 'status' field does not allow filtering.</code></p>
  
  <p>How can I enable filtering on that field?</p>