Added pillar.flask_extra.vary_xhr() decorator

This produces a 'Vary: X-Requested-With' header on the response of
decorated view functions, which indicates to the browser (or intermediate
proxy servers) that the response may/will will be different for XHR and
non-XHR requests.

tests/test_flask_extra.py

@@ -0,0 +1,35 @@
+import unittest
+
+import flask
+
+
+class FlaskExtraTest(unittest.TestCase):
+    def test_vary_xhr(self):
+        import pillar.flask_extra
+
+        class TestApp(flask.Flask):
+            def __init__(self, *args, **kwargs):
+                super().__init__(*args, **kwargs)
+
+                self.add_url_rule('/must-vary', 'must-vary', self.must_vary)
+                self.add_url_rule('/no-vary', 'no-vary', self.no_vary)
+
+            @pillar.flask_extra.vary_xhr()
+            def must_vary(self):
+                return 'yay'
+
+            def no_vary(self):
+                return 'nah', 201
+
+        app = TestApp(__name__)
+        client = app.test_client()
+
+        resp = client.get('/must-vary')
+        self.assertEqual(200, resp.status_code)
+        self.assertEqual('X-Requested-With', resp.headers['Vary'])
+        self.assertEqual('yay', resp.data.decode())
+
+        resp = client.get('/no-vary')
+        self.assertEqual(201, resp.status_code)
+        self.assertNotIn('Vary', resp.headers)
+        self.assertEqual('nah', resp.data.decode())