[4.2] Fixed port separator in DSN when using dblib with PDO in sql server connector

I've been playing with Laravel and SQL server the past few days.

Without this I get a `PDOException` with `SQLSTATE[HY000] Unknown host machine name (severity 2)` on my Mac connecting to SQL server.

I believe it's because dblib is Unix it is expecting a Unix style port separator `:` and the `sqlsrv` option is expecting `,` as it is the Windows style. The `sqlsrv` option works fine when I'm on Windows running Laravel.

Propel ORM has some dblib DSN examples that match too:
http://propelorm.org/Propel/cookbook/using-mssql-server.html#pdodblib