Tidy up suggestion

@jkburges just wanted your feedback on this. 

It removes duplication but uses a function in a way that might be confusing.
ie.
<pre>
var urlFn = this._getUrlGeneratorFunction();

return {
    url: urlFn(collection),
    urlFieldName: this._urlFieldName()
};
</pre>

There might be a different way of declaring the functions so that the scoping works but this usage is clearer?