Enumerator#initialize support duck-type
Use mrb_intern_lit instead of mrb_intern in test driver.
Merge pull request #1974 from ksss/enumerator-duck

Enumerator#initialize support duck-type
Merge pull request #1975 from take-cheeze/driver_mrb_intern_lit

Use mrb_intern_lit instead of mrb_intern in test driver.