net/http/httputil: fix missing previous headers in response when switching protocol in ReverseProxy
When using switching protocol, previous headers set before the reverse proxy are lost.
Fixes #29407
Change-Id: Ia2b9784022d9bccef8625519ccbabbe8a276dfc0
GitHub-Last-Rev:
79bb493dcbb9b76d9d2ff9cd0854b29d634f8b73
GitHub-Pull-Request: golang/go#29408
Reviewed-on: https://go-review.googlesource.com/c/155741
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>