code: public StreamEx<T> append(Collection<? extends T> collection) {
          return appendSpliterator(null, collection.spliterator());
      }
docs: Returns a new {@code StreamEx} which is a concatenation of this stream
  and the stream created from supplied collection.
  
  <p>
  This is a <a href="package-summary.html#StreamOps">quasi-intermediate
  operation</a>.
  
  <p>
  May return this if the supplied collection is empty and non-concurrent.
  
  @param collection the collection to append to the stream
  @return the new stream
  @since 0.2.1