[new] [Experimental] Add support for 3-arity (async) Ring v1.6+ handlers

Motivation:

  It looks like `ring-jetty9-adapter` might not currently be able to
  support async Ajax responses without 3-arity handler support,
  Ref. sunng87/ring-jetty9-adapter#89

  And it looks like it *might* be reasonably easy to add support for
  3-arity handlers to Sente without breaking any pre-existing users.