fix(router): Can return 'handler' param in `'route', $identifier` hook again

In 1.8, it was possible to delegate to a different page handler by registering
a callback for the `'route', $identifier` plugin hook that looks like so:

```php
$result['handler'] = 'other';
return $result;
```

Then during 1.9 it was changed to expect the key to be 'identifier':

```php
$result['identifier'] = 'other';
return $result;
```

This change makes it so that both approaches will work as intended.

Fixes #6696
Mmm... Good point. Would have caught this had I implemented a more
realistic plugin hook... Will try to get to this later today.

On Mon, Jun 23, 2014, 6:20 AM, Steve Clay <notifications@github.com> wrote:

> In engine/classes/Elgg/Router.php:
>
> > @@ -65,7 +65,7 @@ public function route(Elgg_Http_Request $request) {
> >  			return true;
> >  		}
> >
> > -		$identifier = $result['identifier'];
> > +		$identifier = isset($result['identifier']) ? $result['identifier'] : $result['handler'];
>
> This isn't going to work. We need to examine both *for changes*. The hook
> $value will receive both, and in practice most hook handlers will change
> the array and return it as is, so the hook's return will likely *always*
> have both keys.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/Elgg/Elgg/pull/6971/files#r14074195>.
>