restore old struct loading behavior while still avoiding segfaults

rb_obj_alloc() safely creates a properly initialized struct instance, without calling the initialize method of the struct.

This restores pre 2.2.3 behavior. In the future, one could think of make struct loading behavior configurable.

 If so, the code for creating the instance would be:

```c
    parent->val = rb_class_new_instance(len - 1, RARRAY_PTR(value)+1, sc);
```