Add PUT, PATCH, and DELETE to form method attribute

Resolves: whatwg#3577

This adds `put`, `patch`, and `delete` values to the form element's
`method` attribute, and alters navigation to properly handle those
additional HTTP methods. It relies on a corresponding change to fetch
spec, which allows `mode=navigate` fetches to make CORS requests
([fetch spec PR whatwg#1785](whatwg/fetch#1785)).

Author: alexpetros