Size of through association is not correct after adding a has_many association

With the following:

class Post < ActiveRecord::Base
  has_many :readers
  has_many :people, :through => :readers
end

class Person < ActiveRecord::Base
  has_many :readers
  has_many :posts, :through => :readers
end

class Reader < ActiveRecord::Base
  belongs_to :post
  belongs_to :person
end

post = Post.create
post.people << Person.create
post.readers.size        # Returns 2, when 1 is expected.
post.reload.readers.size # Returns 1 as expected.

This happens because ActiveRecord::Associations::HasManyThroughAssociation#insert_record has an extraneous line that adds the same through record after creating it.

A patch with a failing test and the fix is attached.