unable to write binary data to file

This is a blocker.

This can be easily reproduced using

``` ruby
    EM.run do
      EIO.eventmachine_handler # let libeio notify when there's result callbacks to invoke
      EIO.open(path, EIO::RDWR|EIO::CREAT) do |fd|
        EIO.write(fd, "buf contents: \0hello") do |b| # put the zero byte using \0
          EM.stop_event_loop
        end
      end
    end
```
