Looks pretty cool and easy to use, at least for the synchronous methods. The async methods could use a little bit more documentation, e.g. what is really async about them (what is Action? Some kind of deferred response? Can you store the request objects there for longer than the act() methods scope? Etc).

What I would also like to see there are additional examples which show how streaming the request/response body is done, e.g. for file uploads and downloads or SSE.