[1.x] Fixes first class callables namespaces

[nunomaduro]

This pull request fixes the serialisation of first class callable closures. As the full qualified namespaces were not being used on types.

Closes #38.

In other words:

// giving the code
namespace App\Models;
class User { function getUser(): User {} };

// and the serialisation of
$user->getUser(...);

// before 🛑
return function (): User {}

// after ✅
return function (): \App\Models\User {}