Allow to use `HttpRequest` in lambda handlers

[Kaliumhexacyanoferrat]

Hey Chris - Kudos for the API, really feels intuitive.

I am currently adding SimpleW to the HttpArena (see MDA2AV/HttpArena#358) and got some internal server errors that took some time to debug, as I tried to inject the HttpRequest instead of the HttpSession. As an user, it would be nice if I could inject the HttpRequest (so session.Request) into a handler as a shortcut - saves some .Request chains.