Revert "(maint) Use public API when testing binary file #save"

This reverts commit 133c0b8009a48eef77d68c0bd8d5f79c930dab50.
Revert "Merge branch 'pull-2938'"

This reverts commit 1a428b92c900e84d9faa5d0aaa1a32f3c0b1d42b, reversing
changes made to cb26e23706b76e44544c14f4c921885f17dccbb3.
Merge branch 'revert/master/pup-2946-windows-specs'

We're getting spec failures on Windows running Ruby 2.0, we have to
revert this and try again so the build doesn't stay red.