As `#to_s` is defined already in `Object`, basically all object have it. So it would lead warnings about missing path (see my [previous comment](https://github.com/matschaffer/knife-solo/pull/300#issuecomment-26498526)).

Googling lead me to use `:to_str`, which means that the object supports implicit conversion. But as it seems that `String` is the only Ruby core class that implements is, I agree that `.is_a? String` is less confusing. I'll make the change.