Numeric#/ should not be converted by to_f

I know `nil.to_f` is `0.0`.
But this behavior is implicit.
And Current is not implement `coerce`.
(And I don't know why b780e630)

See also
- https://github.com/mruby/mruby/commit/b780e63000f4a2a7d2906c4cbd7fbf9056cbb26b
- https://github.com/mruby/mruby/commit/bd839684e5f6b2810db7c04a015911cfde9aa243