PredicateMatcher recommends changing `#key?` to matcher `be_key`

[bquorning]

This is very old, but the cop still forces some odd changes by default:

- expect(hash.key?(:foo)).to be_truthy
+ expect(hash).to be_key(:foo)

From what I've seen, it's possible to configure matchers, but not to which methods it applies. Couldn't some of these methods common methods even be hardcoded? I don't think that anyone using key? would prefer be_key over key?. And if that'd be the case, with the current implementation it'd be possible to configure it using AllowedExplicitMatchers

Originally posted by @dgmora in #466