Revert "net/http: do not force the Content-Length header if nilled"
This reverts CL 469095.
The newly added TestDisableContentLength is failing on all longtest
builders.
Change-Id: Id307df61c7bf80691d9c276e8d200eebf6d4a59c
Reviewed-on: https://go-review.googlesource.com/c/go/+/495017
Auto-Submit: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>