Use the same behaviour than Ruby 2.3 shellescape

[Morriar]

In it's current state the escape method doesn't accept Pathname instances:

> Shellwords.escape(Pathname.new(""))
NoMethodError:
       undefined method `gsub' for #<Pathname:>

As this gem rewrites the behaviour of the original method this can create surprising behavior:

> [Pathname.new("foo"), "bar"].shelljoin
NoMethodError:
       undefined method `gsub' for #<Pathname:>

This PR fixes this problem by following more closely the original implementation and thus supporting more than a mere string.

[envygeeks]

Thanks 💖