make stderr logic same as original code

In particular, no :stderr option should be treated the same as :merge

In this area, we do have a very small functional difference
which should be noted on the main commit: whereas stderr/stdout
lines could have been interleaved in the past, they are now
always separated.
let -debug show the full info with inspect method

so we will see Pathname objects et al.
more idiomatic Ruby expressions

no functional change
don't do stringification dance in test suite

instead, stub with the real objects that are expected
Merge pull request #6 from rolandwalker/system-command-no-stringification-dance

don't do stringification dance in test suite
Merge pull request #5 from rolandwalker/system-command-idioms

more idiomatic Ruby expressions
Merge pull request #4 from rolandwalker/system-command-inspect-debug

System command inspect debug
Merge pull request #3 from rolandwalker/system-command-stderr

make stderr logic same as original code
test whether :args is an empty array

this is somewhat just for clarity, and somewhat b/c I
have another related change