[2716] Incorrect route generated for page name with underscore

Hi, I found an issue with route generation for pages that contain an underscore in the name.

When page name is pages/sept_2017.vue Observed route path is /sept:2017? Expected route path to be /sept_2017

I'm testing by inspecting this.$nuxt._router.options.routes in the browser:

{path: "/sept:2017?", component: function, name: "sept2017"}

The main problem is that the ? wildcard matches invalid paths like sept_2017njakljsndflkajsndflkajsdf, which breaks 404 routing.

You can work around the problem by removing the underscore from the page filename.