Move request.data to getter method

This change allow to manipulate data with request, in case data being
nested, required modification (a good example could be when instead
delete file we want to change `active` field to false instead -
performing update of { ‘active’ : False } )

rest_framework/mixins.py

@@ -15,8 +15,11 @@ class CreateModelMixin(object):
     """
     Create a model instance.
     """
+    def get_request_data(self, request, *args, **kwargs):
+        return request.data
+
     def create(self, request, *args, **kwargs):
-        serializer = self.get_serializer(data=request.data)
+        serializer = self.get_serializer(data=self.get_request_data(request))
         serializer.is_valid(raise_exception=True)
         self.perform_create(serializer)
         headers = self.get_success_headers(serializer.data)
@@ -62,10 +65,13 @@ class UpdateModelMixin(object):
     """
     Update a model instance.
     """
+    def get_request_data(self, request, *args, **kwargs):
+        return request.data
+
     def update(self, request, *args, **kwargs):
         partial = kwargs.pop('partial', False)
         instance = self.get_object()
-        serializer = self.get_serializer(instance, data=request.data, partial=partial)
+        serializer = self.get_serializer(instance, data=self.get_request_data(request), partial=partial)
         serializer.is_valid(raise_exception=True)
         self.perform_update(serializer)