Through associations should set both parent ids on join models

```ruby
class Member < ActiveRecord::Base
  has_one :current_membership
  has_one :club, through: :current_membership
end

class CurrentMembership < ActiveRecord::Base
  belongs_to :club
  belongs_to :member
end

class Club < ActiveRecord::Base
end

member = Member.new(club: Club.new)
member.save!
```

Before:

```ruby
member.current_membership.club_id # => nil
```

After:

```ruby
member.current_membership.club_id # => club's id
```
Associations are not included in `attributes`.