add Fluent::Config::Section#object_id not to raise NoMethodError by pp or other object dump tools in runtime (logger or other inspection tools may touch this danger spot)
fix to test just existence of object_id
Use alias instead of normal method definition
to suppress warning "redefining `object_id`..."
on Ruby 2.1.x or earlier

See:
 * https://github.com/fluent/fluentd/pull/460
 * https://bugs.ruby-lang.org/issues/10421
  * Issued warning will disapper on coming ruby (2.2.x or later?)