net/http/httputil: clarify the contract on ReverseProxy's Director.

Avoid potential race conditions by clarifying to implemntors of the
ReverseProxy interface, the lifetime of provided http.Request structs.

Fixes #18456
Change-Id: I46aa60322226ecc3a0d30fa1ef108e504171957a
Reviewed-on: https://go-review.googlesource.com/34720Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -30,6 +30,8 @@ type ReverseProxy struct {
 	// the request into a new request to be sent
 	// using Transport. Its response is then copied
 	// back to the original client unmodified.
+	// Director must not access the provided Request
+	// after returning.
 	Director func(*http.Request)
 
 	// The transport used to perform proxy requests.