use existing response.vary property to set vary header

closes #2345

flask/helpers.py

@@ -1004,17 +1004,3 @@ def is_ip(value):
             return True
 
     return False
-
-
-def patch_vary_header(response, value):
-    """Add a value to the ``Vary`` header if it is not already present."""
-
-    header = response.headers.get('Vary', '')
-    headers = [h for h in (h.strip() for h in header.split(',')) if h]
-    lower_value = value.lower()
-
-    if not any(h.lower() == lower_value for h in headers):
-        headers.append(value)
-
-    updated_header = ', '.join(headers)
-    response.headers['Vary'] = updated_header

flask/sessions.py

@@ -19,7 +19,6 @@ from itsdangerous import BadSignature, URLSafeTimedSerializer
 from werkzeug.datastructures import CallbackDict
 from werkzeug.http import http_date, parse_date
 
-from flask.helpers import patch_vary_header
 from . import Markup, json
 from ._compat import iteritems, text_type
 from .helpers import is_ip, total_seconds
@@ -407,7 +406,7 @@ class SecureCookieSessionInterface(SessionInterface):
 
         # Add a "Vary: Cookie" header if the session was accessed at all.
         if session.accessed:
-            patch_vary_header(response, 'Cookie')
+            response.vary.add('Cookie')
 
         if not self.should_set_cookie(app, session):
             return