Test EncodedSring#to_s for undefined conversion / invalid byte sequence

Related to https://github.com/rspec/rspec-core/pull/1760
I don't understand this `REPLACE` proc. Specifically:

* Won't it always return `'?'` (since any string is truthy), so what is the purpose of `|| '� '`?
* Below, I see this proc is explicitly called with an argument (`source_encoding`) but the proc ignores arguments....so what's the point in passing it arguments?
* Given that this operates on no arguments and appears to always return the same value, why is it a proc at all?