net/http: teach NewRequest that NoBody has ContentLength zero

NoBody is new in Go 1.8.

Found while investigating #18117

Change-Id: I6bda030f358e2270f090d108cb3a89c8a2665fcb
Reviewed-on: https://go-review.googlesource.com/33714
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/request.go

@@ -785,7 +785,9 @@ func NewRequest(method, urlStr string, body io.Reader) (*Request, error) {
 				return ioutil.NopCloser(&r), nil
 			}
 		default:
-			req.ContentLength = -1 // unknown
+			if body != NoBody {
+				req.ContentLength = -1 // unknown
+			}
 		}
 		// For client requests, Request.ContentLength of 0
 		// means either actually 0, or unknown. The only way