[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 <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-on: https://go-review.googlesource.com/31437
Run-TryBot: Chris Broadfoot <cbro@golang.org>

src/net/http/clientserver_test.go

@@ -468,7 +468,7 @@ func TestH12_RequestContentLength_Known_NonZero(t *testing.T) {
 }
 
 func TestH12_RequestContentLength_Known_Zero(t *testing.T) {
-	h12requestContentLength(t, func() io.Reader { return strings.NewReader("") }, 0)
+	h12requestContentLength(t, func() io.Reader { return nil }, 0)
 }
 
 func TestH12_RequestContentLength_Unknown(t *testing.T) {