net/http/httputil: don't log read error when it's context.Canceled

Fixes #18838

Change-Id: I44976cadb0dc3c23eacb8cdd58429a572cd8d28a
Reviewed-on: https://go-review.googlesource.com/36358Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -286,7 +286,7 @@ func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int
 	var written int64
 	for {
 		nr, rerr := src.Read(buf)
-		if rerr != nil && rerr != io.EOF {
+		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
 			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
 		}
 		if nr > 0 {