The "After it is called, changing rw.Header will not affect
rw.HeaderMap" claim predates the Result method which changed how the
Recorder should be used.

Fixes #32144
Fixes #32136

Change-Id: I95bdfa5ac489ce7b0202824bb5663f4da188e8a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/178058Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>