net/http: set Cache-Control header only if presents on error
CL 544019 changes http.Error to remove misleading response headers.
However, it also adds new "Cache-Control" header unconditionally, which
may breaks existing clients out there, who do not expect to see the
this header in the response like test in golang.org/x/net/http2.
To keep thing backward compatible, http.Error should only add
Cache-Control header if it has been presented.
Updates #50905
Change-Id: I989e9f999a30ec170df4fb28905f50aed0267dad
Reviewed-on: https://go-review.googlesource.com/c/go/+/569815 Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com> Reviewed-by: Bryan Mills <bcmills@google.com>