Use form.cleaned_data when processing submissions

This fixes #584. It also doesn't pack single values into lists and will
remove csrftoken for us.

wagtail/wagtailforms/models.py

@@ -131,14 +131,8 @@ class AbstractForm(Page):
         return {}
 
     def process_form_submission(self, form):
-        # remove csrf_token from form.data
-        form_data = dict(
-            i for i in form.data.items()
-            if i[0] != 'csrfmiddlewaretoken'
-        )
-
         FormSubmission.objects.create(
-            form_data=json.dumps(form_data),
+            form_data=json.dumps(form.cleaned_data),
             page=self,
         )