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.

pillar/flask_extra.py

@@ -0,0 +1,29 @@
+import functools
+import flask
+
+
+def add_response_headers(headers: dict):
+    """This decorator adds the headers passed in to the response"""
+
+    def decorator(f):
+        @functools.wraps(f)
+        def decorated_function(*args, **kwargs):
+            resp = flask.make_response(f(*args, **kwargs))
+            h = resp.headers
+            for header, value in headers.items():
+                h[header] = value
+            return resp
+
+        return decorated_function
+
+    return decorator
+
+
+def vary_xhr():
+    """View function decorator; adds HTTP header "Vary: X-Requested-With" to the response"""
+
+    def decorator(f):
+        header_adder = add_response_headers({'Vary': 'X-Requested-With'})
+        return header_adder(f)
+
+    return decorator