Don't remove trailing slash from PATH_INFO for mounted apps

Previously when app was mounted as following:

    class Foo
      def call(env)
        [200, {}, [env['PATH_INFO']]]
      end
    end

    RackMountRailsBug::Application.routes.draw do
      mount RackTest.new => "/foo"
    end

trailing slash was removed from PATH_INFO. For example requesting

    GET /foo/bar/

on routes defined above would result in a response containing "/foo/bar"
instead of "/foo/bar/".

This commit fixes the issue.

(closes #3215)