Fix #reload uses id instead of _id

I don't know if it was on purpose, but `Reloadable` looks for the record based on its `id` instead of `_id`.

There's an issue when overriding `#id` alias on another field, to use another ID field for the API: #reload looks for a record where `_id` equals the aliased field, and thus doesn't find the record.

I've just added an underscore in front of every `id` call in reloadable.rb. All specs pass and #reload works when #id is overridden.