net/http: remove misleading response headers on error
This is a reapply of CL 544019 and CL 569815, but with
less aggressive semantics as discussed in proposal #66343.
Error deletes Content-Encoding, since it is writing the response
and any preset encoding may not be correct.
On the error-serving path in ServeContent/ServeFile/ServeFS,
these functions delete additional headers: Etag, Last-Modified,
and Cache-Control. The caller may have set these intending
them for the success response, and they may well not be correct
for error responses.
Fixes #50905.
Fixes #66343.
Change-Id: I873d33edde1805990ca16d85ea8d7735b7448626
Reviewed-on: https://go-review.googlesource.com/c/go/+/571995 Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>