Delegate to core if not mrb_onig_regexp_type

[ksss]

o = Object.new
def o.to_str
  " "
end
"Hello, world!".sub(o, "-")
# Expect(CRuby) => "Hello,-world!"
# Actual => wrong argument type Object (expected Data) (TypeError)

I think, it's more good that mruby-onig-regexp's methods delegate to cores if args are all none OnigRegexp objects instead of string. Then, I think conversion using to_str is core work.

[mattn]

Thank you