Don't loose mysql FOREIGN_KEY_CHECKS status

[gsouf]

Hi,

When truncate operation occurs on mysql database the FOREIGN_KEY_CHECKS is modified and when restored, it is set to an arbitrary ("1") value and the previous value is lost.

In some case we use this trick to allow phpunit to populate database from dataset without foreign key errors:

protected function setUp()
    {
        $conn=$this->getConnection();
        $conn->getConnection()->query('set FOREIGN_KEY_CHECKS=0;');
        parent::setUp();
        $conn->getConnection()->query('set FOREIGN_KEY_CHECKS=1;');
    }

The truncate operations conflicts with this and this PR aims to fix that.

[gsouf]

See #37

[sebastianbergmann]

I merged this pull request without testing it. I do not use DbUnit myself and trust the existing tests as well comments on the pull requests that this does not break anything and solves a problem.