net/http: add Request.RequestURI field

net/http: add Request.RequestURI field

The new url.URL's parsing can be too canonicalizing for
certain applications. By keeping the original request URI
around, we give applications a gross escape hatch while
keeping the URL package clean and simple for normal uses.

(From a discussion with Gary Burd, Gustavo Niemeyer,
and Russ Cox.)

Fixes #2782

R=golang-dev, rsc, dsymonds
CC=golang-dev
https://golang.org/cl/5580044