Prevent encoding errors when visiting a page in an encoding node doesn't recognize

Zombie will throw an encoding error if a page's charset is set to an encoding that Node's `toString` method doesn't recognize (https://github.com/joyent/node/blob/master/lib/buffer.js#L438-L463), such as iso-8859-1. This change checks a page's charset against those encodings, and won't pass an encoding to `toString` if it's not one of node's valid encodings, letting it fall through to utf-8.

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