Bugfix on inherit private magic methods

You should probably add a test to https://github.com/Ocramius/ProxyManager/blob/10a871f89a2a0c4a69930dbfece1486afd8e124b/tests/ProxyManagerTest/Functional/MultipleProxyGenerationTest.php#L103 instead.

Also, let's use a test asset instead of `PDO`, as `ext-pdo` is not in `require-dev` for this package.