def write_to_string_io(string_io)
      java_proto.write_to(org.jruby.util.IOOutputStream.new(string_io))
      nil
    end
def write_to_string_io(string_io)
      java_proto.write_to(org.jruby.util.IOOutputStream.new(string_io))
      nil
    end
Serialize the message to the given StringIO.

 @return [nil]