Update explicit_subject.feature

Created by: jackrg

It took me some hunting to figure out that when you just want to invoke the subject so that you can test side effects of the call, then you can just use "subject.call". This amends the section on explicit subjects to add a new scenario. The example is admittedly a little out there, but I think that it demonstrates the sorts of things you might use this for. This is my first PR for anything, so if I've violated any customs or protocols, I offer my apologies.