Made it possible to explicitly bypass check for nested writes

This allows for reusing create() and update() methods of ModelSerializer class when one wants to handle nested writes, but does not want to reimplement or copy the code that is already there in these methods. So, in a subclass of ModelSerializer, one can just call the parent's create() or update() method with check_for_nested_writes=False.

rest_framework/serializers.py

@@ -737,7 +737,7 @@ class ModelSerializer(Serializer):
 
     # Default `create` and `update` behavior...
 
-    def create(self, validated_data):
+    def create(self, validated_data, check_for_nested_writes=True):
         """
         We have a bit of extra checking around this in order to provide
         descriptive messages when something goes wrong, but this method is
@@ -758,7 +758,8 @@ class ModelSerializer(Serializer):
         If you want to support writable nested relationships you'll need
         to write an explicit `.create()` method.
         """
-        raise_errors_on_nested_writes('create', self, validated_data)
+        if check_for_nested_writes:
+            raise_errors_on_nested_writes('create', self, validated_data)
 
         ModelClass = self.Meta.model
 
@@ -797,8 +798,9 @@ class ModelSerializer(Serializer):
 
         return instance
 
-    def update(self, instance, validated_data):
+    def update(self, instance, validated_data, check_for_nested_writes=True):
-        raise_errors_on_nested_writes('update', self, validated_data)
+        if check_for_nested_writes:
+            raise_errors_on_nested_writes('update', self, validated_data)
 
         for attr, value in validated_data.items():
             setattr(instance, attr, value)