[HttpKernel] enabled cache-reloading when cache file is rebuilt

This allows the cache file to be deleted and thus rebuilt between Container builds in WebTestCase scenario, to enable testing of multiple configurations of the same application through WebTestCase.

src/Symfony/Component/HttpKernel/Kernel.php

@@ -482,12 +482,19 @@ protected function initializeContainer()
         if (!$cache->isFresh()) {
             $container = $this->buildContainer();
             $container->compile();
+            for ($classVersion = 0; class_exists($class . ($classVersion ?: '')); $classVersion++);
+            $class .= $classVersion ?: '';
             $this->dumpContainer($cache, $container, $class, $this->getContainerBaseClass());
 
             $fresh = false;
+        } else {
+            for ($classVersion = 0; class_exists($class . ($classVersion ?: '')); $classVersion++);
+            $class .= $classVersion ? $classVersion - 1 : '';
         }
 
-        require_once $cache->getPath();
+        if(!class_exists($class)) {
+            require $cache->getPath();
+        }
 
         $this->container = new $class();
         $this->container->set('kernel', $this);