Change OutputCacheHelper to non static

Author: lanlanlee2008

src/OutputCacheModuleAsync/HttpRequestHeaders.cs

@@ -7,5 +7,12 @@ static class HttpRequestHeaders {
         public const string CacheControl = "Cache-Control";
         public const string Pragma = "Pragma";
         public const string Range = "Range";
+        public const string HeaderServer = "HttpWorkerRequest.HeaderServer";
+        public const string HeaderSetCookie = "HttpWorkerRequest.HeaderSetCookie";
+        public const string HeaderCacheControl = "HttpWorkerRequest.HeaderCacheControl";
+        public const string HeaderExpires = "HttpWorkerRequest.HeaderExpires";
+        public const string HeaderLastModified = "HttpWorkerRequest.HeaderLastModified";
+        public const string HeaderEtag = "HttpWorkerRequest.HeaderEtag";
+        public const string HeaderVary = "HttpWorkerRequest.HeaderVary";
     }
 }