FactoryGirl should refuse to assign to a private setter.

As part of a refactoring, I'm making one of the association setters on one of my ActiveRecord models private, something like:

```ruby
class Human
belongs_to :favorite_soda, class_name: :Soda
private :favorite_soda=
```

But FactoryGirl will still gladly assign to it:

```ruby
FactoryGirl.build(:human, favorite_soda: Soda.first)
```

This is because the `AttributeAssigner` uses `#send` instead of `#public_send`. This PR changes that.

I haven't added a test for it because I wasn't sure where it should be tested. `AttributeAssigner` doesn't appear to have its own spec (though I might just not be seeing it).

This change doesn't break any specs, but it's possible that users are depending on FactoryGirl being able to assign to private setters. I think it's unlikely they're intending to do that, though. At least, I can't come up with a reason you'd want to make something private but then assign it from a factory.

Thoughts?