title: Django ModelAdmin get_form() doesn't set fields attribute
body: <p>I've overriden <code>get_form()</code> in my <code>ModelAdmin</code> class:</p>
  
  <pre><code>def get_form(self, request, obj=None, **kwargs):
      form = super(ModelAdmin, self).get_form(request, obj, **kwargs)
  </code></pre>
  
  <p>Now, if I add this line:</p>
  
  <pre><code>print form.fields
  </code></pre>
  
  <p>I get an:</p>
  
  <pre><code>AttributeError: type object 'FilerImageForm' has no attribute 'fields'
  </code></pre>
  
  <p>Why is this happening? Should the call to the super <code>get_form()</code> set the <code>fields</code> attribute on the form? If I am mistaken, how can I access fields on a form in a <code>ModelAdmin</code> class?</p>