I have found a solution myself, without modifying functions.php
You just need to add
redirect 404 /robots.txt to .htaccess and then remove the /robots.txt using URL removal tool from google webmaster tools.
I am doing so because i think robots.txt is counter-productive. I noticed i lost traffic each time i added disallowed folders to robots.txt
I don't have documents to hide.
On the other hand, matt cutt said we should not block folders where there is .js
Then, .swf are not indexed by wordpress.
So i think it is useless to block /wp-includes/ or /wp-content/
At last, i think it is useless to block /wp-admin/ and /wp-includes/ because spam bots don't read robots.txt anyways.