Changed @_children to @__children

As-is, when calling Changeable#move_changes, it would overwrite the @_children
instance variable with nil. This caused a children *relation* to return nil
after the parent was saved.

  class Tree
    include Mongoid::Document

    belongs_to :parent,   :class_name => "Tree", :inverse_of => :children
    has_many   :children, :class_name => "Tree", :inverse_of => :parent
  end

> instance = Tree.new
> instance.children
  => []
> instance.save
  => true
> instance.children
  => nil

> parent = Tree.new
> parent.save
  => true
> child = Tree.new
> child.parent = parent
  => NoMethodError: undefined method `push' for nil:NilClass
     from /lib/mongoid/relations/bindings/referenced/in.rb:29:in
     `block in bind_one'