[5.7] Make ResourceCollection countable such that it works with count()

[michaeldyrynda]

I ran into an issue with a Laravel 5.5 project that I've been upgrading to 5.7 and moving to PHP 7.2.

For consistent representation of my models, I transform them using resources via a presenter and when passing a resource collection to the Blade @each directive, I was hitting a count(): Parameter must be an array or an object that implements Countable error.

This PR makes ResourceCollection implement the Countable interface so that it can be passed straight into @each, which worked fine before the breaking change in 7.2.