Return an error in Accept() when root is closed.

As reported in #4, when the root listener is closed, calling Accept
on mux'd listeners would block forever, instead of returning an error.
This is because of a bug introduced in b90740d.

This commit fixes #4 by selecting on both donec and connc of muxed
listeners.

Added a test case to guard against this issue.

cmux.go

@@ -158,11 +158,12 @@ type muxListener struct {
 }
 
 func (l muxListener) Accept() (c net.Conn, err error) {
-	c, ok := <-l.connc
-	if !ok {
+	select {
+	case c = <-l.connc:
+		return c, nil
+	case <-l.donec:
 		return nil, ErrListenerClosed
 	}
-	return c, nil
 }
 
 type MuxConn struct {