title: Path XX contains invalid priority: Must be a String, Double, ServerValue, or Null
body: <p>I'm trying to set a value to a child using the server ServerValue.TIMESTAMP. </p>
  
  <pre><code>    val userRef = database.reference.child(userId).child("cache")
      val beerRef = userRef.child("beers")
      beerRef.child(beer.id).setValue(beer)
      val beerExpiration = userRef.child("expiration/beer")
      val serverTimeStamp = mapOf(Pair("timestamp", ServerValue.TIMESTAMP))
      beerExpiration.child(beer.id).setValue("timestamp", serverTimeStamp)
  </code></pre>
  
  <p>I'm receiving this error:</p>
  
  <pre><code>io.reactivex.exceptions.OnErrorNotImplementedException: Path '/xxxxxxxx/cache/expiration/beer/4010' contains invalid priority: Must be a string, double, ServerValue, or null
      at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:704)
      at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:701)
      at io.reactivex.internal.observers.LambdaObserver.onError(LambdaObserver.java:77)
      at io.reactivex.internal.observers.LambdaObserver.onNext(LambdaObserver.java:67)
      at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.drainNormal(ObservableObserveOn.java:200)
      at io.reactivex.internal.operators.observable.ObservableObserveOn$ObserveOnObserver.run(ObservableObserveOn.java:252)
      at io.reactivex.android.schedulers.HandlerScheduler$ScheduledRunnable.run(HandlerScheduler.java:109)
      at android.os.Handler.handleCallback(Handler.java:751)
      at android.os.Handler.dispatchMessage(Handler.java:95)
      at android.os.Looper.loop(Looper.java:154)
      at android.app.ActivityThread.main(ActivityThread.java:6776)
      at java.lang.reflect.Method.invoke(Native Method)
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
  </code></pre>