Allow indexed non-empty arrays being valid parameter values

Currently, there is no way to store indexed arrays as values except direct call `$container->set(...)`:
```
DI\Definition\Exception\DefinitionException: Invalid key '0' in definition of entry 'foo'; Valid keys are: class, scope, lazy, constructor, properties, methods
```
By the way, it's impossible to discern type of parameter (simple value or service) in case of empty array.

P.S. I didn't add this feature for `XmlDefinitionFileLoader`, anyway it looks not fully supported at this moment.