HAL::Links module shouldn't prepend an underscore to links

According to the [docs](https://github.com/apotonick/roar/blob/master/lib/roar/representer/json/hal.rb#L84) the links key should not have an underscore. That should be added when the HAL module in mixed in and overrides the links_definition_options.

I prefer the hal-like syntax for the links key, but w/o the underscore.