RxScala: Fix infinite recursive onStart call in Subscriber

[jbripley]

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)

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[zsxwing]

Good catch. LGTM.

[benjchristensen]

Please also fix this in the new RxScala project.