Allow described_class in specs

[toddmohney]

described_class makes it easier to change the name/namespace of the class. (think 'make it easy to do the right thing' where the 'right thing' here is refactoring to a better name)

Lowering the barrier to this type of change will make it easier for us to correct domain debt.

[maxjacobson]

I'm personally on board with it but curious to hear why people weren't in the past.

Do you think we should prescribe that we do use it, or just allow that we may use it?

If we want to prescribe it, we should start using rubocop-rspec, which has a cop for this: https://github.com/backus/rubocop-rspec/blob/master/lib/rubocop/cop/rspec/described_class.rb

[pbrisbin]

I'm OK with allowing folks that want to to use described_class. I don't think we should prescribe that they must.