net/http: make timeout errors match context.DeadlineExceeded

net/http: make timeout errors match context.DeadlineExceeded

When returning an error which implements net.Error and reports
itself as a timeout, also report it as matching context.DeadlineExceeded.
This matches the behavior of timeout errors in the net package
and elsewhere.

Fixes #50856

Change-Id: I2ca911e3677a699af27ba89b1200401baa8b3b1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/567537
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>