RxScala: Fix infinite recursive onStart call in Subscriber

Without this fix, when subscribing using a `new Subscriber`, onStart would be called in an infinite loop. Giving a strack trace similar to this:

```
java.lang.StackOverflowError
	at rx.lang.scala.SubscriberTests$$anon$1.onStart(SubscriberTests.scala:65)
	at rx.lang.scala.Subscriber$$anon$1.onStart(Subscriber.scala:8)
	at rx.lang.scala.Subscriber$class.onStart(Subscriber.scala:43)
	at rx.lang.scala.SubscriberTests$$anon$1.onStart(SubscriberTests.scala:65)
	at rx.lang.scala.Subscriber$$anon$1.onStart(Subscriber.scala:8)
	at rx.lang.scala.Subscriber$class.onStart(Subscriber.scala:43)
```