Renamed private properties in behat events to support Symfony's Serializer calls.

Problem happens when you try to serialize behat events with Symfony Serializer. Default behavior of serializer (with GetSetMethodNormalizer()) assumes that object's attribute names match getters, which is not true for some of the behat events.

Since, there's no big deal in how the private properties are named, I've prepared a pull request, which makes all of them match their getters.