Remove Enumerator#enum_with_index on 1.9 and later
Remove duplicated Enumerator#with_index definition on 1.9 and later
Add a Enumerator::Lazy#{to_enum,enum_for} spec
Fix Enumerator::Lazy#{to_enum,enum_for}

Fixes #2585
Merge pull request #2586 from kachick/issue-2585

Fix #2585