save_to_memory closes stream so you can't use it later

> ...
> output = StringIO()
> book = pe.Book({'hoja1':[['datos', 'de', 'prueba'],[1, 2, 3]], })
> book.save_to_memory('csv', output)
> logger.debug(output.getvalue())
> This will raise an exception "I/O operation on closed file"
