Do not timeout when WriteControl deadline is zero

A zero value for the Conn.WriteControl deadline specifies no timeout,
but the feature was implemented as a very long timeout (1000 hours).
This PR updates the code to use no timeout when the deadline is zero.

See the discussion in #895 for more details.

conn.go

@@ -438,14 +438,14 @@ func (c *Conn) WriteControl(messageType int, data []byte, deadline time.Time) er
 		maskBytes(key, 0, buf[6:])
 	}
 
-	d := 1000 * time.Hour
-	if !deadline.IsZero() {
-		d = time.Until(deadline)
+	if deadline.IsZero() {
+		// No timeout for zero time.
+		<-c.mu
+	} else {
+		d := time.Until(deadline)
 		if d < 0 {
 			return errWriteTimeout
 		}
-	}
-
 		select {
 		case <-c.mu:
 		default:
@@ -457,6 +457,7 @@ func (c *Conn) WriteControl(messageType int, data []byte, deadline time.Time) er
 				return errWriteTimeout
 			}
 		}
+	}
 
 	defer func() { c.mu <- struct{}{} }()
 

conn_test.go

@@ -148,6 +148,22 @@ func TestFraming(t *testing.T) {
 	}
 }
 
+func TestWriteControlDeadline(t *testing.T) {
+	t.Parallel()
+	message := []byte("hello")
+	var connBuf bytes.Buffer
+	c := newTestConn(nil, &connBuf, true)
+	if err := c.WriteControl(PongMessage, message, time.Time{}); err != nil {
+		t.Errorf("WriteControl(..., zero deadline) = %v, want nil", err)
+	}
+	if err := c.WriteControl(PongMessage, message, time.Now().Add(time.Second)); err != nil {
+		t.Errorf("WriteControl(..., future deadline) = %v, want nil", err)
+	}
+	if err := c.WriteControl(PongMessage, message, time.Now().Add(-time.Second)); err == nil {
+		t.Errorf("WriteControl(..., past deadline) = nil, want timeout error")
+	}
+}
+
 func TestConcurrencyWriteControl(t *testing.T) {
 	const message = "this is a ping/pong messsage"
 	loop := 10