WIP fix for kw args issue - DO NOT MERGE

See https://github.com/rspec/rspec-mocks/pull/726
So this skips the positional args (arity) check, right?  I think that for this case where there's a matcher we should just treat the keyword args in the signature as an additional positional arg (required if there are any required keyword args, otherwise optional), and then still perform the `valid_non_kw_args?` comparison.