fix a bug in respond_to_key which cannot generate a unique key for different klass and sel

The same key is generated in the following two cases.

1. klass = 0xfd085b0
    sel = 0xfd3cf90
    (0xfd085b0 + 0xfd3cf90  -> 0x1FA45540)

2. klass = 0xfd085d0
    sel = 0xfd3cf70
    (0xfd085d0 + 0xfd3cf70 -> 0x1FA45540)

  => generated key = 1068693925

Object#respond_to? is not working as expected, because the keys that are not unique.