Fix crash if nil passed to NewTickerWithTimer

Doc for function says that passing nil for the timer argument should
cause the default system timer to be used.  Ensure that that timer is
actually assigned.

ticker.go

@@ -33,6 +33,9 @@ func NewTicker(b BackOff) *Ticker {
 // NewTickerWithTimer returns a new Ticker with a custom timer.
 // A default timer that uses system timer is used when nil is passed.
 func NewTickerWithTimer(b BackOff, timer Timer) *Ticker {
+	if timer == nil {
+		timer = &defaultTimer{}
+	}
 	c := make(chan time.Time)
 	t := &Ticker{
 		C:     c,

ticker_test.go

@@ -87,3 +87,10 @@ func TestTickerContext(t *testing.T) {
 		t.Errorf("invalid number of retries: %d", i)
 	}
 }
+
+func TestTickerDefaultTimer(t *testing.T) {
+	b := NewExponentialBackOff()
+	ticker := NewTickerWithTimer(b, nil)
+	// ensure a timer was actually assigned, instead of remaining as nil.
+	<-ticker.C
+}