Fix inherited stdout/stderr when PosixSpawnProcess does not have an `@io`.

PosixSpawnProcess only set io streams if `@io` exists which is not the case if the process creator never calls `#io`. Therefor the stdout and stderr are inherited by default.

This fix changes the if conditions to use the `io` getter method that provides a lazy initialization for Unix::IO.
Add spec for io inheritance

Also add a spec helper to capture process' own stdout and stderr.
Require ostruct for mri <2.0 and JRuby
Merge pull request #72 from jgraichen/spawn-process-io

Fix default inherited stdout/stderr when PosixSpawnProcess does not have an @io.