update the test custom provider's add method to match the legacy behavior

Author: lanlanlee2008

test/CustomOutputCacheProvider/CustomOutputCacheProvider.cs

@@ -64,7 +64,7 @@ public override async Task<object> AddAsync(string key, object entry, DateTime u
                 _dict.Add(key, new CustomOutputCacheItem(entry, utcExpiry));
             }
             else {
-                _dict[key] = new CustomOutputCacheItem(entry, utcExpiry);
+                entry = _dict[key].Obj;
             }
             return entry;
         }
@@ -127,7 +127,7 @@ public override object Add(string key, object entry, DateTime utcExpiry) {
                 _dict.Add(key, new CustomOutputCacheItem(entry, utcExpiry));
             }
             else {
-                _dict[key] = new CustomOutputCacheItem(entry, utcExpiry);
+                entry = _dict[key].Obj;
             }
             return entry;
         }