Improve `_exec` and its test coverage

I'm not sure that we want to unwrap all `.apple-style-span` `span`s. Could have such a span been merged with a span that a user would like to keep, perhaps? From some plugin that uses spans for something, perhaps?

My current code removes spans if they lack an `id`, a class or their only class is `apple-style-span`. This seems reasonable to me but there may be other reasons to keep a `span`, like your `data-` suggestion. I can't really tell until I see such a span created. I remember that PhantomJS creates `.apple-style-span`s so it may be possible to debug using it.