Fix for flushQueue db fetchAll

[jchen-privco]

While testing this in development of the email utility feature I discovered the original fork's flushQueue did not fetch the serialized emails from the database as intended. Using PDO::FETCH_COLUMN resulted in a unserialize failure.

[doublecompile]

Hi, @jchen-privco. Thanks for the pull request! If you could please ensure the unit tests pass, I can accept this. Currently, all of the Travis CI builds fail.