check type of token property instead of using in operator

This makes it more convenient to define knox clients like:

```javascript
var client = knox.createClient({
    key: '<api-key-here>'
  , secret: '<secret-here>'
  , bucket: 'learnboost'
  , token: tokenVariableMightBeUndefined // w/ this patch, headers['x-amz-security-token'] will not get set if this variable here is undefined.
});
```