make private method enumerator_block_call
modify core method that return Enumerator object if not block given
fix return value for with_index
fix self modifying bug
test split and to detail
fix variable name typo
optimize from each to enumerator_block_call
add test for Array#each_index if non block given
Merge pull request #1859 from ksss/enumerator

Fix Enumerator