fix(API): Always set the Vary header when serializing errors (#952)

falcon/api_helpers.py

@@ -123,7 +123,6 @@ def default_serialize_error(req, resp, exception):
             preferred = 'application/xml'
 
     if preferred is not None:
-        resp.append_header('Vary', 'Accept')
         if preferred == 'application/json':
             representation = exception.to_json()
         else:
@@ -132,6 +131,8 @@ def default_serialize_error(req, resp, exception):
         resp.body = representation
         resp.content_type = preferred + '; charset=UTF-8'
 
+    resp.append_header('Vary', 'Accept')
+
 
 def wrap_old_error_serializer(old_fn):
     """Wraps an old-style error serializer to add body/content_type setting.

tests/test_httperror.py

@@ -311,6 +311,7 @@ class TestHTTPError(testing.TestBase):
 
         body = self.simulate_request('/fail', headers=headers)
         self.assertEqual(self.srmock.status, headers['X-Error-Status'])
+        self.assertEqual(self.srmock.headers_dict['Vary'], 'Accept')
         self.assertEqual(body, [])
 
     def test_custom_old_error_serializer(self):