fix BC for #6890

This is reorganize of https://github.com/zendframework/zf2/pull/6890 and https://github.com/zendframework/zf2/commit/c91bd9be56e0b194beedde4c78be943591cdc719.

Because ```defaultPlatform``` is realy BC and don't required in ```Zend\Db\Sql\Platform\*``` classes

Why do you need `is_a()` here?