[release-branch.go1.7] net/http: update test to check Content-Length 0 Body more reliably
[release-branch.go1.7] net/http: update test to check Content-Length 0 Body more reliably

The way to send an explicitly-zero Content-Length is to set a nil Body.

Fix this test to do that, rather than relying on type sniffing.

Updates #17480
Updates #17071

Change-Id: I6a38e20f17013c88ec4ea69d73c507e4ed886947
Reviewed-on: https://go-review.googlesource.com/31434
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>
Reviewed-on: https://go-review.googlesource.com/31437
Run-TryBot: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go