Container::set after a value has been set and retrieved

``` php
$container->set('foo', 'bar');
$container->get('foo');
// bar

$container->set('foo', 'baz');
$container->get('foo');
// bar (should be baz)
```

The value is "cached" in the `$entries` array, and `set` only add a new definition but doesn't update the `$entries` array.

Fix should be trivial.
