assert an error is caused by JSON parsing for rails 2.x.x

assert_json_parse_exception.rb

  def assert_json_parse_exception(error)
    if ActiveSupport::JSON.respond_to?(:parse_error) # 2.3.5
      parse_error_class = ActiveSupport::JSON.parse_error
      assert_instance_of parse_error_class, error
    else
      assert_instance_of ActiveSupport::JSON::ParseError, error
    end
  end