GateController::logRequest refactoring

Summary:
Any gate without any specific requirement can call logRequest
without argument.

Other gates can pass an array with extra contextual data to log.

The log format doesn't change.

Test Plan: tail -n 10 -f storage/logs/laravel.log && phpunit

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D219