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