When updating the in-memory entity value, replace

We were not actually replacing the item in the update case.

Author: kzu

src/TableStorage.Memory/MemoryRepository`1.cs

@@ -167,7 +167,7 @@ public async Task<T> PutAsync(T entity, CancellationToken cancellation = default
             (key, value) =>
             {
                 key.Timestamp = DateTimeOffset.UtcNow;
-                return SetTimestamp(value, key.Timestamp);
+                return SetTimestamp(entity, key.Timestamp);
             }
         );