Restore kind_of to match using kind_of? rather than ===

Restore `kind_of(...)` to match using `kind_of?` rather than `===`

Fixes #744.
What about changing `numeric` to `numeric_klass`? It took me a moment to realize that's what this was referring too. The variable names are `numeric` and `fix_num` both class names, but `fix_num` is meant to represent an instance and not the class.