Revert "Create new pinger from default one"

This reverts commit 0bf1ce5.

pinger.go

@@ -68,7 +68,7 @@ type Pinger struct {
 
 // NewPinger constructs new Pinger instance optionally with additional options.
 func NewPinger(options ...PingerOption) Pinger {
-	pinger := defaultPinger
+	var pinger Pinger
 	for _, configure := range options {
 		configure(&pinger)
 	}