Enumrable#each_with_index return Enumerator if non block given
 Add Enumerable#sort_by test
Add Enumerable#first test
Reduce implicit type casts.
rescue StopIteration only.
Merge pull request #1884 from monaka/pr-reduce-implicit-type-cast

Reduce implicit type casts.
Merge pull request #1886 from iij/pr-kernel-loop-rescue-error

Kernel.loop should not catch StandardError.
Merge pull request #1883 from suzukaze/add-test-enumerable

Add enumerable test
Merge pull request #1880 from ksss/enum-each_with_index

Enumrable#each_with_index return Enumerator if non block given