Small code change to allow fallback/default policies

[shawnheide]

I've been trying to implement default policies and noticed that Nova handles policies slightly different to the Laravel framework. In the policy checks in Laravel, they use is_callable to check if a policy class has a method. Nova uses method_exists which doesn't allow the __call method to be used in a policy class.

In order to allow for things like fallback or default policies, Nova would need to replace the method_exists calls in Authorizable.php with is_callable. I tested this out locally and it seems to work without any issues (so far). Here is the regex for replacing if you are curious: replace all method_exists\(\$gate, (.+?)\) to is_callable([$gate, $1]) in the Authorizable trait in Nova.

Let me know if you have any questions. Thanks for your time in considering this change.

Related issue: #1147

[crynobone]

Released with Laravel Nova 5.0.0

Feel free to open up a new issue if you're still experiencing this problem on the latest version.