Do not assume that `spec/filters` are Drop filters

[dark-panda]

We have a project that has some classes in app/filters and consequently spec/filters that have nothing to do with Liquid, but the liquid-rails gem makes the assumption that whatever is in a filters directory relates to Drop filters. This should probably be a manual configuration option that you need to explicitly make in your RSpec configuration rather than something that is done automagically, as it is a simple configuration process and does not need to happen implicitly.