Fix paginator options when disabled.

Fix a paginator bug (or at least unexpected behavior) when using disabled options and the prev/next links being disabled.

Currently

    $this->Paginator->prev('<i class="fa fa-angle-left"></i>', array('escape' => false), null, array('class' => 'prev disabled'));

would display just fine (non-escaped) for active links. Once it is disabled, it will escape suddenly.
I think the general opinion would be that $disabledOptions can extend/overwrite the normal $options, not having to repeat all options again there.

This resolves it.

Thanks for Patrick for pointing it out to me earlier.

Also fixing return type to "empty string" where null was returned.