title: Django form validation fails (on update)
body: <p>I'm fighting with the Django form validation for a while now. I'm using pretty basic code. Returning the form data using an ajax request is the only special thing but I don't think its the cause auf my problems here.
  I'm trying to update an user entry. I create an instance of the user and I feed that instance into the CreateForm() but I still get the duplicate username error.</p>
  
  <p>here is a part of the view:</p>
  
  <pre><code>def user_update(request):
      if request.is_ajax():
          print request.POST
          user = User.objects.get(pk=int(request.POST['pk']))
          print user
          form = UserCreateForm(request.POST, instance=user)
          print form
          if form.is_valid():
              form_cleaned = form.cleaned_data
          else:
              print '### form is invalid ###'
              print form.error_messages
  </code></pre>
  
  <p>The output for <code>request.POST</code> is:</p>
  
  <pre><code>&lt;QueryDict: {u'username': [u'testuser'], u'password1': [u'test'], u'password2': [u'test'], u'pk': [u'27'], u'csrfmiddlewaretoken': [u'wyBoaBAlxLTO952BzWSxR7HMK6W7nsAM'], u'email': [u'soso@soso.so']}&gt;
  </code></pre>
  
  <p>The output for <code>print user</code> is:</p>
  
  <pre><code>testuser
  </code></pre>
  
  <p>In <code>print form</code> I always get the duplicate username error, hence the form is always invalid.</p>
  
  <p>Please give me a hint. Many thanks in advance.</p>