use the correct encoding in the `decodeKludges` method

If I recall correctly, `header.kludges` is [a Node.js Buffer.](https://github.com/askovpen/node-fidonet-squish/blob/c65412262d35c101486dfc2f42e085342ba85c3b/fidonet-squish.js#L134)

By performing [`RegExp.prototype.exec`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec) on that Buffer you're implicitly calling `header.kludges.toString()` (without parameters) and [it defaults](http://nodejs.org/docs/latest/api/buffer.html#buffer_buf_tostring_encoding_start_end) to the `'utf8'` encoding (instead of the necessary `encoding` value that you determined above).

This pull request introduces an explicit `.toString` encoding and thus eliminates the problem.