Support optional fields in resource object

[bxh]

Per JSON API spec, attributes, relationships and links are optional. However, in Saule they are always provided. This could cause unexpected behaviors for JSON API clients such as Ember Data. And although link is a neat feature, it takes up additional space. I'm wondering if the community here is willing to make those fields optional like included?

[joukevandermaas]

I'm very much in favor of leaving out optional keys when they are empty. Sounds like a good feature to me.