Fix a spec for return value of Enumerator#{each_}with_object

"given object" depends #equal?, not #==
Fix a spec for last block parameter of Enumerator#{each_}with_object
Improve specs for Enumerator#{each_}with_object
Merge pull request #195 from kachick/improve-shared-enumerator-with_object

Improve specs for Enumerator#{each_}with_object