Use an empty string for SCRIPT_NAME.

Rack's `path` method returns `script_name + path_info`, see [here](https://github.com/rack/rack/blob/master/lib/rack/request.rb#L331)

If we set both variables to uri.path we end up with a path repeated when calling the `path` method.

```ruby
env = {"SCRIPT_NAME" => "/foo/bar", "PATH_INFO" => "/foo/bar"}
req = Rack::Request.new(env)
req.path #=> "/foo/bar/foo/bar"
```

According to Rack [SPEC](https://github.com/rack/rack/blob/master/SPEC#L24), we should set this to the path were our application was mounted, but I'm not sure if we could know that.

Setting this value to an empty string make Rack's `path` method work as expected.