Update PdoServiceProvider.php

[VladimirMerk]

Fixes a bug

With multiple connections with different drivers for each instance, a connection is created with arguments from the last element of the array.

Example:
$app->register(
new LiquidBox\Silex\Provider\PdoServiceProvider(null, 'pdo.dbs'),
array(
'pdo.dsn' => array(
'mysql_main' => array(
'connection' => array(
'host' => 'mysql_host',
'dbname' => 'msqlexample',
),
'username' => 'user',
'password' => 'pass',
),
'postgres_main' => array(
'driver' => 'pdo_pgsql',
'connection' => array(
'host' => 'postgres_host',
'dbname' => 'psgexample'
),
'username' => 'user',
'password' => 'password',
)
)
)
);

Result:
instances[mysql_main] = pgsql:host=postgres_host;dbname=psgexample
instances[postgres_main] = pgsql:host=postgres_host;dbname=psgexample