Update SearchForm.php

Hello,

I think there is a little problem at line 258.
The third argument of cache->set() is a ttl, so $expiration must be set to a ttl, and not to a absolute timestamp. This can lead to caching problems, because ttl is set to 2014 - 1970 + 10 years (from cache-control header) = 50 years, that is a too high ttl for some caching systems.

Cheers,

Pierre