nothing, it's used in RSpec 3 though. I was torn between copying over the implementation (what I've done), or adding a minimal class that makes it work. We do need to ensure one parameter is passed, otherwise any change would be incompatible. Actually, I should be calling `to_s` before passing up to `super`.

Given people will be switching to this constant, and may be looking for the implementation, I erred on the side of including redundant code.