Mutant specs

ran mutant through the code.

Added tests +122/-21
minor tweaks to the code +10/-18

only 1 remaining issue regarding `:in` parameter. will put into separate PR

/cc @jrafanie
Lol, I was laughing at this.

The other commit simplified multiple lines into a single line, but kept all the same logic.
This commit removed the unneeded logic.

I went back in and verified `output` and `error` are not nil, even when the stderr is redirected.

Just added a spec to make that testing explicit