Index out of bounds exception when encountering "Invalid mac."

Hi,

I'm using tent/hawk-objc on the client side and tent/hawk-ruby on the server side. 

Receiving the following message causes an index out of bounds exception
in https://github.com/tent/hawk-objc/blob/master/Hawk/HawkAuth.m#L229 :

```
Hawk error="Invalid mac. hawk.1.header, 1412010586, ewUfzUUd, GET, /v1/all?page=1&per_page=50, api.myapp.com, 80, "
```

Cheers,
Frane
