Fix nested associations after a change in 0.10.0

[mwoskowicz]

Let's say we have a model

class User < ApplicationRecord
has_one :logo, class_name: "UsersArea::Logo"
accepts_nested_attributes_for :logo
end

In the 0.10.0 it tries to constantize a class based on the association name itsself, which breaks if association name does not correspond directly to the class name. In this example it tries to find class Logo instead of UsersArea::Logo

This change seems to have fixed that problem for us at least. It would be great if someone could verify in their project :)