Use Buffer.byteLength instead of msg.length

When logging utf-8 data the module currently responds with the following error:

``` js
client.log({hellå:wårld});
// --> { [Error: Parse Error] bytesParsed: 166, code: 'HPE_INVALID_CONSTANT' }
```

This PR fixes this by using `Buffer.byteLength(msg)` instead of `msg.length` for content-length.