insert() should not alter the row value

This is a bug in current master

It converts alpha to binary, which is then saved to cache.  Only
alpha number should be saved in cache, test case:

1. purge cache.
2. post some new posts.
3. purge cache.
4. post some new posts.
5. the topic_list cache contains both alpha and binary values

Change-Id: Icfa4279c3e93b65eba6dc408a49247a566c7a7e7