Fix skip paths so ".js" doesn't skip ".json"

While using ruby-prof to test a Rack-based REST API, I noticed it wasn't profiling any of my requests. After some digging, it turned out that the regex being applied to skip javascript requests (".js") was unintentionally skipping my requests for paths that ended in ".json". I changed the regex to be more specific so that the path had to *end* with ".js", which now allows ".json" to proceed.

I didn't see any existing tests which test the middleware, but let me know if I just missed it and I'll update accordingly.