elixir-lang/elixir — pull request #1084 by mururu (2013-05-17 00:22:32+00:00)
Title: Fix capture_io behaviour when receiving :stdio

:erlang.whereis(:standard_io) returns :undefined and the io lib of erlang replace :standard_io with group_leader()(https://github.com/erlang/otp/blob/maint/lib/stdlib/src/io.erl#L553).
So capture_io should treat :standard_io in the same way as :group_leader.