[release-branch.go1.21] net/http: send body or close connection on expect-100-continue requests

When sending a request with an "Expect: 100-continue" header,
we must send the request body before sending any further requests
on the connection.

When receiving a non-1xx response to an "Expect: 100-continue" request,
send the request body if the connection isn't being closed after
processing the response. In other words, if either the request
or response contains a "Connection: close" header, then skip sending
the request body (because the connection will not be used for
further requests), but otherwise send it.

Correct a comment on the server-side Expect: 100-continue handling
that implied sending the request body is optional. It isn't.

For #67555
Fixes #68199

Change-Id: Ia2f12091bee697771087f32ac347509ec5922d54
Reviewed-on: https://go-review.googlesource.com/c/go/+/591255


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
(cherry picked from commit cf501e05)
Reviewed-on: https://go-review.googlesource.com/c/go/+/595096


Reviewed-by: Joedian Reid <joedian@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>