Align Write timeout handling with Read timeout handling

In hashicorp#31, the Read path was changed to move away from
time.After. This change was not reflected in the Write path, and this
commit rectifies that.

stream.go

@@ -220,9 +220,11 @@ START:
 
 WAIT:
 	var timeout <-chan time.Time
+	var timer *time.Timer
 	writeDeadline := s.writeDeadline.Load().(time.Time)
 	if !writeDeadline.IsZero() {
 		delay := writeDeadline.Sub(time.Now())
+		timer = time.NewTimer(delay)
 		timeout = time.After(delay)
 	}
 	select {
@@ -231,6 +233,9 @@ WAIT:
 	case <-timeout:
 		return 0, ErrTimeout
 	}
+	if timer != nil {
+		timer.Stop()
+	}
 	goto START
 }