title: Migrating ManyToManyField to null true, blank true, isn't recognized
body: <p>I have made a model change from </p>
  
  <pre><code>standard = models.ManyToManyField(Standard)
  </code></pre>
  
  <p>to</p>
  
  <pre><code>standard = models.ManyToManyField(Standard, blank=True, null=True)
  </code></pre>
  
  <p>South schemamigration for this app doesn't recognize the change? </p>
  
  <p>Similar to this question, which is unanswered:
  <a href="https://stackoverflow.com/questions/13755119/south-migrations-and-changes-to-many-to-may-fields">South migrations and changes to many-to-may fields</a></p>