stream: be less eager with readable flag

As of 34b535f4, test-child-process-flush-stdio was failing
on CentOS 5 systems in CI due to the change in stream state
checking in `child_process`. This commit fixes those failures
by making readable streams less eager in setting their readable
flag on EOF.

Fixes: https://github.com/nodejs/node/issues/4125
PR-URL: https://github.com/nodejs/node/pull/4141


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>