Tested and fixed on 0.10.33

With this change, tests passed with node 0.10.33.
You've opened in append mode https://github.com/jib/piped/blob/master/test/05_local_listen.js#L101 so on Linux this makes sense given:
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On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.
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