fix the cache regression issue #10 and #19

Author: lanlanlee2008

src/OutputCacheModuleAsync/OutputCacheHelper.cs

@@ -413,7 +413,7 @@ public async Task CacheResponseAsync() {
             if (settings.SlidingExpiration) {
                 slidingDelta = settings.SlidingDelta;
             }
-            else if (settings.MaxAge != TimeSpan.MinValue) {
+            else if (settings.MaxAge != TimeSpan.Zero) {
                 DateTime utcTimestamp = (settings.UtcTimestampCreated != DateTime.MinValue)
                     ? settings.UtcTimestampCreated
                     : _context.Timestamp;