Method registry should include cacheability #54

HTTP Methods have 3 boolean properties, all of which 8.1.2 says a registration needs to define, but only "Safe" and "Idempotent" were included in the registry while "Cacheable" was omitted.

I filed this as https://www.rfc-editor.org/errata/eid5300, but it probably makes more sense as a change for ter.