title: Add request user to InlineFormSet: TypeError: __init__() got an unexpected keyword argument 'user'
body: <p>I'm using <code>InlineFormSetView</code> from <code>extra_views</code> to create a custom inlineform. 
  On form save I need to make some changes that's relevant to the request user. So the request user is required in the form. From what I found, I came up with this solution but it seems that <code>get_formset_kwargs</code> may be used elsewhere. Causing an exception to occur. I'm not quite sure what is the cause. May be there is another solution to adding request user to an inline form.</p>
  
  <p><strong>View:</strong></p>
  
  <pre><code>class MyModelSetsView(InlineFormSetView):
      model = MyModel
      inline_model = MySubModel
      form_class = MySubModelSetForm
      template_name = "update_sets.html"
      success_message = "Updated successfully."
      permission_required = []
      factory_kwargs = {
          'extra': 1,
      }
  
      def get_success_url(self):
          return self.object.get_absolute_url()
  
       def get_formset_kwargs(self):
           kwargs = super(MyModelSetsView, self).get_formset_kwargs()
           kwargs.update({'user': self.request.user})
           return kwargs
  </code></pre>
  
  <p><strong>Form:</strong></p>
  
  <pre><code>class MySubModelSetForm(forms.ModelForm):
      def __init__(self, *args, **kwargs):
          self.user = kwargs.pop('user', None)
          super(PeakPeriodSetForm, self).__init__(*args, **kwargs)
  </code></pre>
  
  <p><strong>Exception:</strong></p>
  
  <pre><code> File "..\venv\lib\site-packages\extra_views\formsets.py", line 268, in get
      formset = self.construct_formset()
    File "...venv\lib\site-packages\extra_views\formsets.py", line 36, in construct_formset
      return formset_class(**self.get_formset_kwargs())
    File "...\venv\lib\site-packages\django\forms\models.py", line 897, in __init__
      super().__init__(data, files, prefix=prefix, queryset=qs, **kwargs)
    File "...\venv\lib\site-packages\django\forms\models.py", line 569, in __init__
      super().__init__(**{'data': data, 'files': files, 'auto_id': auto_id, 'prefix': prefix, **kwargs})
  TypeError: __init__() got an unexpected keyword argument 'user'
  </code></pre>