Using `/` in output.filename no longer works

[SpaceK33z]

Originally from webpack/webpack-dev-server#670 - @ibufu

Previously, using a / in output.filename worked:

output: {
        path: path.resolve(__dirname, "dist"),
        filename: '/[name].js'
},

Now it returns a 404.

This probably broke when the GetFilenameFromUrl method was refactored. Should add an extra test for this.

[SpaceK33z]

Confirmed that this is not an issue in webpack-dev-middleware, but in webpack. See webpack/webpack#3189