Fixed unserialization for closure-style step definitions

When you use Behat with Gearman extension, you might get "Serialization of 'Closure' is not allowed" exception in case if one of your steps is implemented like this: http://docs.behat.org/guides/5.closures.html

This happens, since German processes communicate between each other with serialized classes. When such serialization happens with a result of a step, defined in closure, you'll get a problem.