Cannot read property 'indexOf' of undefined

Problem comes at [L904 of /dist/raven.](https://github.com/getsentry/raven-js/blob/master/dist/raven.js#L904) Sometimes xhr requests are made without `url`.

A simple fix might be:

``` js
if (url && url.indexOf(self._globalKey) === -1) {
```

or

``` js
url = url || "";

if (url.indexOf(self._globalKey) === -1) {
```
