fix cache expiration issue when the url is passed via options for the mo...

This pull request solves a problem of trying to save a model with a different endpoint from the common url. Since the method getCacheKey is not getting the options for the request, it tries to expire the cache for the models url, which is  none, if the model is not configured with 'rootUrl' and it throws error:

"Uncaught Error: A "url" property or function must be specified "

```javascript
myModel = new MyModel({ attr: 'x' })
myModel.save({}, { url: '/different-end-point-only-for-this-request'})
```