Make Url.resolve include format in the returned value

I've noticed that `Iron.Url.resolve` would drop periods when placed right before named parameters. This was caused by not including `format` argument (i.e. the period) in the returned value. The following example illustrates the issue and the fix:

```javascript

var url = new Url('/posts/:id.:format');

// Before this change:
url.resolve({id: 123, format: 'html'}); // => "/posts/123html"

// After this change:
url.resolve({id: 123, format: 'html'}); // => "/posts/123.html"