code: @CheckReturnValue
      @NonNull
      public static <T> BehaviorRelay<T> createDefault(T defaultValue) {
          return new BehaviorRelay<T>(defaultValue);
      }
docs: Creates a {@link BehaviorRelay} that emits the last item it observed and all subsequent items to each
  {@link Observer} that subscribes to it.
  
  @param defaultValue
  the item that will be emitted first to any {@link Observer} as long as the
  {@link BehaviorRelay} has not yet observed any items from its source {@code Observable}