net/http: use reflect.TypeFor for known types

net/http: use reflect.TypeFor for known types

For #60088

Change-Id: I9e4044d9c2694fe86aab1f5220622c8d952b1a90
Reviewed-on: https://go-review.googlesource.com/c/go/+/522338
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>