Add ActiveRecord information

Although we can use `Bugsnag::MetaData`, it is useful if bugsnag automatically detect ActiveRecord related error such as `ActiveRecord::RecordNotSaved` and report the attributes.
https://github.com/rails/rails/blob/5142d5411481c893f817c1431b0869be3745060f/activerecord/lib/active_record/errors.rb#L55