Fix typo in client; AuthDrivers with getStateParam method

If an AuthDriver implements it's own `getStateParam` method you run into this error:

```
[...]/node_modules/dropbox/lib/dropbox.js:1169
                  if (_this.client.authStep === DbxClient.RESET) {
                                  ^
TypeError: Cannot read property 'authStep' of undefined
```

I assume it's just a typo since a few lines earlier there is this switch statement

```coffee
      switch @authStep
        when DbxClient.RESET
```

I tried to write a unit test for this, but that would lead into a duplication of ['completes the authenticate flow'](https://github.com/dropbox/dropbox-js/blob/b4444be01ecb5189632406ff0b843050745a6ba6/test/src/client_test.coffee#L1798), which is way to much overhead.

Please drop me a line when you have any idea how to test this more elegantly.

Cheers.