Fixes SingleItemObservable to emit item only once

[gael-ft]

Hi MongoDB, @rozza,

I observed what seems to be a tiny bug.
It is about the SingleItemObservable class.
This class is called in SingleObservable.scala file

object SingleObservable {

  /**
   * Creates an SingleObservable from an item.
   *
   * Convenient for testing and or debugging.
   *
   * @param item the item to create an observable from
   * @tparam A the type of the SingleObservable
   * @return an Observable that emits the item
   */
  def apply[A](item: A): SingleObservable[A] = SingleItemObservable(item)
}

I figured it out when adding tests and mocking some DB calls, and returning SingleObservable(aTestValue), I always got an exception :

java.lang.IllegalStateException: SingleObservable.onNext cannot be called with multiple results.
	at org.mongodb.scala.ObservableImplicits$ToSingleObservablePublisher$$anon$1.check(ObservableImplicits.scala:125)
	at org.mongodb.scala.ObservableImplicits$ToSingleObservablePublisher$$anon$1.onNext(ObservableImplicits.scala:114)
        ...

As a quick solution I have to use Observable(aTestValue :: Nil).toSingle, which is aware of wether it was already completed. This PR is meant to add this into the SingleItemObservable class.

I did not create a JIRA ticket for this, as it is a very tiny issue, but I can if necessary.

Thanks !

[rozza]

Thanks @gael-ft