Fix polyfill support issue in CSRF validator

- typehint on AbstractContainer
- when instantiating a Container instance, use fully qualified class name in
  order to trigger alias

library/Zend/Validator/Csrf.php

@@ -11,7 +11,7 @@
 
 use Traversable;
 use Zend\Math\Rand;
-use Zend\Session\Container as SessionContainer;
+use Zend\Session\AbstractContainer as SessionContainer;
 use Zend\Stdlib\ArrayUtils;
 
 class Csrf extends AbstractValidator
@@ -174,7 +174,8 @@ public function setSession(SessionContainer $session)
     public function getSession()
     {
         if (null === $this->session) {
-            $this->session = new SessionContainer($this->getSessionName());
+            // Using fully qualified name, to ensure polyfill class alias is used
+            $this->session = new \Zend\Session\Container($this->getSessionName());
         }
         return $this->session;
     }