code: public static <T extends Serializable> Flowable<T> write(final Flowable<T> source, final File file) {
          return write(source, file, false, DEFAULT_BUFFER_SIZE);
      }
docs: Writes the source stream to the given file in given append mode and using
  the a buffer size of 8192 bytes.
  
  @param source
  flowable stream to write
  @param file
  file to write to
  @param <T>
  the generic type of the input stream
  @return re-emits the input stream