ActiveSupport broken on UTF-8 strings

json.rb

# encoding: UTF-8

# parsing with EOF throws illegal characters!?
json = <<EOF
{
  "date_a": "2011-07-22T09:54:44Z",
  "city": "wroc\u0142aw",               # after this the next date is not parsed anymore!
  "date_b": "2011-07-22T09:54:44Z" 
}
EOF

# this json works
json2 = ' { "date_a": "2011-07-22T09:54:44Z", "city": "wroc\u0142aw", "date_b": "2011-07-22T09:54:44Z" }'

require 'active_support/json'
require 'oj'
ActiveSupport.parse_json_times = true
p ActiveSupport::JSON.backend
p ActiveSupport::JSON.decode(json2)

It parses date value into ruby DateTime object which is invalid for mongodb ruby driver.