:inverse_of not honored when replacing a has_one association

When assigning to, or calling #replace, on a has_one association, the inverse association is not set.

The patch attached fixes this.

require 'active_record'

ActiveRecord::Base.establish_connection('adapter' => "sqlite3", 'database' => ":memory:")
ActiveRecord::Schema.define do
  create_table :parents do
  end
  create_table :children do |t|
    t.integer :parent_id
  end
end

class Parent < ActiveRecord::Base
  has_one :child, :as => :parent, :inverse_of => :parent
end

class Child < ActiveRecord::Base
  belongs_to :parent, :inverse_of => :child
end

parent = Parent.new(:child => Child.new)
new_child = Child.new
parent.child = new_child

p parent.child.parent.target.equal?(parent)  # should be true