Mutations in the resource are addressed by changes in the media type. You're talking about it as if it was rocket science, while everything is very well addressed.

That's Best Practices for Designing an HTTP API. Of 22 points, the author deals with HTTP implementation details in 14, and REST is supposed to be protocol agnostic and leverage on the standard. In 6 points he goes directly against the REST principles, and in 2 other points he directly contradicts them.

Sorry, but if you follow his guide, you won't have a RESTful API at all. I don't understand how can someone sell that as a "best practices" manual.