Fix for issue ReactiveX#1494 "Infinite recursion in SerialDisposable.disposable getter."

Author: serges147

RxSwift/Disposables/SerialDisposable.swift

@@ -34,7 +34,7 @@ public final class SerialDisposable : DisposeBase, Cancelable {
     public var disposable: Disposable {
         get {
             return _lock.calculateLocked {
-                return self.disposable
+                return _current ?? Disposables.create()
             }
         }
         set (newDisposable) {