WIP fix for kw args issue - DO NOT MERGE

See https://github.com/rspec/rspec-mocks/pull/726
WDYT of this API? Could also move to `valid?`, but that's a bigger refactoring since we use it in `split_args`. Given it is a stateful object anyway, I think it makes most sense here - easy to code, and there is no need for flexibility of having it on `valid?` since we reconstruct this object anyway.