http2: set ContentLength to -1 for HEAD response with no Content-Length

When reading a response to a HEAD request with no Content-Length,
set the Response ContentLength to -1 (unknown) rather than 0.

Fixes failing net/http TestH12_HeadContentLengthNoBody.

Change-Id: I472cbcf555e7f70bc41f58bf96da3d3c5bed9654
Reviewed-on: https://go-review.googlesource.com/c/net/+/360376
Trust: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: neild

http2/transport.go

@@ -2291,7 +2291,7 @@ func (rl *clientConnReadLoop) handleResponse(cs *clientStream, f *MetaHeadersFra
 	} else if len(clens) > 1 {
 		// TODO: care? unlike http/1, it won't mess up our framing, so it's
 		// more safe smuggling-wise to ignore.
-	} else if f.StreamEnded() {
+	} else if f.StreamEnded() && !cs.isHead {
 		res.ContentLength = 0
 	}