http2: set Response.ContentLength to 0 when headers end stream

When reading a response with no Content-Length and no body, set the
Response ContentLength to 0.

Fixes failing net/http TestH2_204NoBody and TestH2_304_NoBody tests.

Change-Id: I8ded6f8febfce62e00f2c37de65cbb09b306f7c6
Reviewed-on: https://go-review.googlesource.com/c/net/+/359834
Trust: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: neild

http2/transport.go

@@ -2291,6 +2291,8 @@ 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() {
+		res.ContentLength = 0
 	}
 
 	if cs.isHead {