net/http: handle WriteHeader(101) as a non-informational header
Prior to Go 1.19 adding support for sending 1xx informational headers
with ResponseWriter.WriteHeader, WriteHeader(101) would send a 101
status and disable further writes to the response. This behavior
was not documented, but is intentional: Writing to the response
body explicitly checks to see if a 101 status has been sent before
writing.
Restore the pre-1.19 behavior when writing a 101 Switching Protocols
header: The header is sent, no subsequent headers are sent, and
subsequent writes to the response body fail.
For #59564
Change-Id: I72c116f88405b1ef5067b510f8c7cff0b36951ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/485775 Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>