net/textproto: reject HTTP requests with empty header keys
According to RFC 7230, empty field names in HTTP header are invalid.
However, there are no specific instructions for developers to deal
with that kind of case in the specification. CL 11242 chose to skip
it and do nothing about it, which now seems like a bad idea because
it has led `net/http` to behave inconsistently with the most widely-used
HTTP implementations: Apache, Nginx, Node with llhttp, H2O, Lighttpd, etc.
in the case of empty header keys.
There is a very small chance that this CL will break a few existing HTTP clients.
Fixes #65244
Change-Id: Ie01e9a6693d27caea4d81d1539345cf42b225535
Reviewed-on: https://go-review.googlesource.com/c/go/+/558095 Reviewed-by: Bryan Mills <bcmills@google.com> Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>