SQLite db:drop:all tiny fix + more teeny tinies

Hi all,

Just some tiny fixes looking for reviews before the new year:

http://dev.rubyonrails.org/ticket/10577 - this one is for the db:drop:all task, so that it doesn't silently ignore errors removing SQLite databases. http://dev.rubyonrails.org/ticket/10621 - docfix for incorrect example for assert_valid_keys http://dev.rubyonrails.org/ticket/10631 - another tiny cleanup to the db:sessions:clear rake task

Thanks for your time again!

Cheers, Chu Yeow