windows redirect problems

[micosty]

if (options.rootPath !== '/') {
    options.loginPath = path.join(options.rootPath, options.loginPath);
    options.logoutPath = path.join(options.rootPath, options.logoutPath);
    options.loginCallbackPath = path.join(options.rootPath, options.loginCallbackPath);
  }

https://github.com/koajs/userauth/blob/master/index.js#L65-L69

path.join在windows下是反斜杠，登录跳转地址有误，出现重定向循环

[fengmk2]

Same bug as node-modules/userauth#11, I will fix this soon.