Autowiring should ignore optional parameters

[mnapoli]

from #168

class Foo
{
    public function __construct(Bar $bar = null)
    {
        $this->bar = $bar ?: $this->createDefaultBar();
    }
}

Today, by default PHP-DI will inject a Bar instance (because of autowiring). It should instead inject the default value, unless specified otherwise.

Fixing this will be a small BC break, but I think it's a necessary evil since it's quite problematic.

[mnapoli]

Fixed in 4.2