client.authenticate in browser with browserify

I encountered an error with `client.authenticate`. I tried to use the function in the browser inside a browserify bundle. The error is thrown in browser.js:157:
`Uncaught TypeError: Object [object Object] has no method 'getResponseHeader'`.  `[object Object]` referes to the request object.
I figured that's because hawk assumes `request` to be the native xhr object. When using browserify, a node like request object will be passed to the function. The problem is that the node equivalent function to `getResponseHeader` is called `getHeader`. I fixed that by checking which function is available.