Fix child process sending UTF8.

Currently the child process messages are decoded as ASCII, which breaks when UTF8 characters are stringified. As mentioned in this StackOverflow question:
http://stackoverflow.com/questions/15378871/node-js-child-process-send-utf-8-char-issue

I'm happy to add more to this like tests if someone points me in the right direction. Or if there was a reason this was ascii, maybe there is some other solution.

Basic repro steps:
```
var n = require('child_process').fork(...);
n.send('中文');
```