Effective Go has references to a function call f(c, req) made by ServeHTTP mixed with f(w,
req). c is dropped in favor of w to maintain consistency

Fixes #17128

Change-Id: I6746fd115ed5a58971fd24e54024d29d18ead1fa
Reviewed-on: https://go-review.googlesource.com/29311Reviewed-by: Rob Pike <r@golang.org>