Don't select if providing default database

By default, the database is 0, so there is no need to select a database if '0' is provided.

This also makes php-resque-ex compliant with twemproxy, which does not support the 'select' command.
adding {} to if statement for PSR compliance
Merge pull request #8 from pvwoods/master

Performing a DB select when the DB is set to default '0' is not necessary and breaks Twemproxy