Add arena save/restore around use of hash KEY.
Refactor mrb_hash_has_keyWithKey() func in hash.c
Merge pull request #1307 from carsonmcdonald/keyduparenafix

Add arena save/restore around use of hash KEY.
Merge pull request #1308 from suzukaze/refactor-hash-c

Refactor mrb_hash_has_keyWithKey() func in hash.c