Service manager and canonicalization

With my custom plugin manager :

```php
class OverrideKeysPluginManager extends AbstractPluginManager
{
    protected $invokableClasses = array(
        'FOO' => 'Foo',
    );
}
```

Because 'foo' use upper case, I can't get my service with the key 'foo', because overridden keys does not use the canonicalization.