use existing response.vary property to set vary header

closes #2345

tests/test_basic.py

@@ -9,20 +9,21 @@
     :license: BSD, see LICENSE for more details.
 """
 
-import pytest
-
-import re
-import uuid
-import time
-import flask
 import pickle
+import re
+import time
+import uuid
 from datetime import datetime
 from threading import Thread
-from flask._compat import text_type
-from werkzeug.exceptions import BadRequest, NotFound, Forbidden
+
+import pytest
+import werkzeug.serving
+from werkzeug.exceptions import BadRequest, Forbidden, NotFound
 from werkzeug.http import parse_date
 from werkzeug.routing import BuildError
-import werkzeug.serving
+
+import flask
+from flask._compat import text_type
 
 
 def test_options_work(app, client):
@@ -523,14 +524,14 @@ def test_session_vary_cookie(app, client):
     @app.route('/vary-cookie-header-set')
     def vary_cookie_header_set():
         response = flask.Response()
-        response.headers['Vary'] = 'Cookie'
+        response.vary.add('Cookie')
         flask.session['test'] = 'test'
         return response
 
     @app.route('/vary-header-set')
     def vary_header_set():
         response = flask.Response()
-        response.headers['Vary'] = 'Accept-Encoding, Accept-Language'
+        response.vary.update(('Accept-Encoding', 'Accept-Language'))
         flask.session['test'] = 'test'
         return response