Apply default scope when joining associations.

For example:

    class Post < ActiveRecord::Base
      default_scope -> { where published: true }
    end

    class Comment
      belongs_to :post
    end

When calling `Comment.join(:post)`, we expect to receive only
comments on published posts, since that is the default scope for
posts.

Before this change, the default scope from `Post` was not applied,
so we'd get comments on unpublished posts.

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/associations/join_dependency/join_association.rb