[5.0] Added default URL to back() method

If the referer is missing, the app goes down...

This proposal modifies the current way to use the back method.
I think the suggestion was just to make it: `if ( ! $back) $back = $default;`

Alternatively; `$back = $this->generator->getRequest()->headers->get('referee') ?: $default;` But I'm not sure how Taylor feels about ternaries.

Other than that, :+1: as this has bitten me a few times.