title: django makemigration alterfield without any change
body: <p>On every makemigrations altered estate field while don't change that field. what's reason of it?!</p>
  
  <pre><code>class Announcement(OJModel):
  owner = models.ForeignKey('user.User', verbose_name=_('Announcement Owner'), on_delete=models.CASCADE,
                            related_name='owner_announcements', null=True, blank=True)
  title = models.CharField(max_length=300, default='', verbose_name=_('Title'))
  description = models.TextField(max_length=3000, default='', verbose_name=_('Description'))
  estate = models.OneToOneField('announcement.Estate', on_delete=models.CASCADE, verbose_name=_('Estate'), null=True)
  </code></pre>
  
  <p>migration script generated:</p>
  
  <pre><code>class Migration(migrations.Migration):
  
  dependencies = [
      ('announcement', '0072_auto_20181218_1256'),
  ]
  
  operations = [
      migrations.AlterField(
          model_name='announcement',
          name='estate',
          field=models.OneToOneField(null=True, on_delete=django.db.models.deletion.CASCADE, to='announcement.Estate', verbose_name='Estate'),
      ),
  
  ]
  </code></pre>