net/http/httptest: deprecate ResponseRecorder.HeaderMap
net/http/httptest: deprecate ResponseRecorder.HeaderMap

Users of this field are better off using Result instead.

Fixes #25763.

Change-Id: I4391afa6ed3873107628630adc1d409d77fb3f20
Reviewed-on: https://go-review.googlesource.com/117675
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,josharian/go,josharian/go,josharian/go,josharian/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,golang/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,josharian/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,cloudflare/go,golang/go,golang/go,josharian/go,nawawi/go