net/http: second do not force the Content-Length header if nilled

net/http: second do not force the Content-Length header if nilled

This is a second round of CL 469095 which has been fixed after
the issue discovered in the revert CL 495017.

The issue was a missing res.Body.Close() in the newly added test.

Change-Id: Ifd9d8458022e59f4486397443a2862d06383e990
Reviewed-on: https://go-review.googlesource.com/c/go/+/495115
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Jorropo <jorropo.pgm@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>