LearnBoost/knox — pull request #143 by ianshward (2013-02-14 20:11:53+00:00)
Title: 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: ''
, secret: ''
, bucket: 'learnboost'
, token: tokenVariableMightBeUndefined // w/ this patch, headers['x-amz-security-token'] will not get set if this variable here is undefined.
});
```