Only add 'Vary: Accept' header when there is more than one possible renderer.

rest_framework/views.py

@@ -112,11 +112,13 @@ class APIView(View):
 
     @property
     def default_response_headers(self):
-        # TODO: Only vary by accept if multiple renderers
-        return {
+        headers = {
             'Allow': ', '.join(self.allowed_methods),
-            'Vary': 'Accept'
         }
+        if len(self.renderer_classes) > 1:
+            headers['Vary'] = 'Accept'
+        return headers
+
 
     def http_method_not_allowed(self, request, *args, **kwargs):
         """