net/http: fix send on close channel error

Fixes #3793.

Tested using GOMAXPROCS=81 which was able to trigger a panic
in TestStressSurpriseServerCloses continually on a Core i5.

R=fullung, bradfitz
CC=golang-dev
https://golang.org/cl/6445069

src/pkg/net/http/transport.go

@@ -538,7 +538,6 @@ func remoteSideClosed(err error) bool {
 
 func (pc *persistConn) readLoop() {
 	defer close(pc.closech)
-	defer close(pc.writech)
 	alive := true
 	var lastbody io.ReadCloser // last response body, if any, read on this connection
 
@@ -640,7 +639,9 @@ func (pc *persistConn) readLoop() {
 }
 
 func (pc *persistConn) writeLoop() {
-	for wr := range pc.writech {
+	for {
+		select {
+		case wr := <-pc.writech:
 			if pc.isBroken() {
 				wr.ch <- errors.New("http: can't write HTTP request on broken connection")
 				continue
@@ -653,6 +654,9 @@ func (pc *persistConn) writeLoop() {
 				pc.markBroken()
 			}
 			wr.ch <- err
+		case <-pc.closech:
+			return
+		}
 	}
 }