net/http: attempt deadlock fix in TestDisableKeepAliveUpgrade

1. The test now checks the response status code.
2. The transport has been changed to not set "Connection: Close" if
   DisableKeepAlive is set and the request is a HTTP/1.1 protocol
   upgrade.

Updates #43073

Change-Id: I9977a18b33b8747ef847a8d11bb7b4f2d8053b8c
GitHub-Last-Rev: f809cebb139df4f5560a8456973351c95a3dfa97
GitHub-Pull-Request: golang/go#43086
Reviewed-on: https://go-review.googlesource.com/c/go/+/276375


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>