Adding MySQL2 MULTI_STATEMENTS flag

I tested this patch locally after running into `<Sequel::DatabaseError> Mysql2::Error: PROCEDURE <proc> can't return a result set in the given context` errors when running stored procedures via Sequel. All sequel tests also pass.

See https://github.com/brianmario/mysql2/issues/208 for background on the Mysql2::Client::MULTI_STATEMENTS flag.