Tire: STI field _type get's corrupted by update_index callback on second call to save

Tire-_type-field-corruption.rb

require 'mongo_mapper'
MongoMapper.database = 'testing'
require 'tire'

class Thing
  include MongoMapper::Document
  include Tire::Model::Search
  include Tire::Model::Callbacks
end

class SubThing < Thing 
end

s = SubThing.create # => #<SubThing _id: BSON::ObjectId('5076d365d92b09213d000001'), _type: "sub_thing"> 
# _type is actually persisted as SubThing as you can see with a reload
s._type # => "sub_thing"
SubThing.count # => 1
s.save # => true
s._type # => "sub_thing"
SubThing.count # => 0 !!!

This is referred to by an issue on the tire gem: karmi/retire#477