Omitting null values

[Schoonology]

Rather than just submit a PR, I thought it wise to discuss potential solutions first.

I just started replacing a lot of custom JSON-API-consuming Swift code with Spine, and noticed an issue. The server that I'm integrating with is using swagger-tools, which means that, unfortunately, a value cannot be null, it must either be the appropriate type (e.g. a String) or omitted. It looks like SerializeOperation.addAttributes supports this behavior via the OmitNullValues option, but I don't see anywhere in the public API where this option is exposed, nor any internal implementation that uses it.

For the time being, I have a local version of Spine where that option is always provided. What is the right way to expose this behavior to consumers?