4.x - Inconsistent setBasePath Behavior

[l0gicgate]

• When using $app->setBasePath('/public') it breaks RouteParser::relativeUrlFor output:

  echo $routerParser->urlFor('about');
  /public/about
  

  echo $routerParser->relativeUrlFor('about');
  about <- expected
  /public/about <- result
  

• Using only $app->getRouteCollector()->setBasePath('/public'), RouteParser produces corrent urls for both urlFor and relativeUrlFor methods but breaks current path detection

• Using both $app->setBasePath and $routeCollector->setBasePath produces invalid url paths for all methods

Originally posted by @piotr-cz in #2512 (comment)

[piotr-cz]

I have prepared test case here: Slim-4-basepath-issue