Fix capture_io behaviour when receiving :stdio or :standard_io

`:erlang.whereis(:standard_io)` returns `:undefined` and the io lib of erlang replaces `:standard_io` with `group_leader()`(see 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`.