fixed failed at set

Fixed problem about update existing value.
![screenshot from 2014-05-03 09 47 36](https://cloud.githubusercontent.com/assets/1883569/2869317/ed065ec8-d264-11e3-9433-d4da1abb227d.png)
HOWEVER, this fix will ignore command input about overwrite. 
TODO: a further fix is necessary,  
Further fix should focus on  bin/stash:383 , maybe a check about arg before ask,.  I don't know how should I deal with arg so leave it here.