code: public static <T> Transformer<T, T> doOnFirst(final Action1<? super T> action) {
          return doOnNext(1, action);
      }
docs: Returns a {@link Transformer} that applied to a source {@link Observable}
  calls the given action on the first onNext emission.
  
  @param action
  is performed on first onNext
  @param <T>
  the generic type of the Observable being transformed
  @return Transformer that applied to a source Observable calls the given
  action on the first onNext emission.