Ignore None when passed in as response override

Closes #148

src/drf_yasg/inspectors/view.py

@@ -230,6 +230,8 @@ class SwaggerAutoSchema(ViewInspector):
                 response = openapi.Response(
                     description=serializer
                 )
+            elif not serializer:
+                continue
             elif isinstance(serializer, openapi.Response):
                 response = serializer
                 if hasattr(response, 'schema') and not isinstance(response.schema, openapi.Schema.OR_REF):

src/drf_yasg/utils.py

@@ -76,6 +76,8 @@ def swagger_auto_schema(method=None, methods=None, auto_schema=unset, request_bo
 
         * if a plain string is given as value, a :class:`.Response` with no body and that string as its description
           will be generated
+        * if ``None`` is given as a value, the response is ignored; this is mainly useful for disabling default
+          2xx responses, i.e. ``responses={200: None, 302: 'something'}``
         * if a :class:`.Schema`, :class:`.SchemaRef` is given, a :class:`.Response` with the schema as its body and
           an empty description will be generated
         * a ``Serializer`` class or instance will be converted into a :class:`.Schema` and treated as above