Pass the XHR object along `page:load`

This provides us with more information about the new page, such as its headers and URL.  This can be used to integrate, for example, header-based JavaScript dispatch logic:

```ruby
before_filter :set_action

private def set_action
  headers['X-Action'] = action_name
end
```

```javascript
document.addEventListener('page:load', function(event) {
  var xhr = event.data;
  if (xhr.getResponseHeader('X-Action') === 'edit') {
    alert("You're editing something");
  }
});
```