Saner keyword arguments support

Apologies for the previous implementation, it worked more by accident than anything else (it required the existence of a method named the same was as the argument, which happened to be the case in all of my specs…).

This allows the kwargs to be anything, which is still sub-optimal¹, but better than what’s on master at the moment.

¹ ideally, the stringifier would default them to their actual default values – but I failed to find a stable way to do it with the method_source gem; the other idea was to parse `Ripper.sexp` output fed with `Method#source_location`, but that might be more work than it’s worth ;)