[Session] Fixed a bug with the TestListener

When the session is not started, the test listener would still save the
session causing the session data to be emptied.

src/Symfony/Bundle/FrameworkBundle/EventListener/TestSessionListener.php

@@ -68,7 +68,9 @@ class TestSessionListener implements EventSubscriberInterface
         }
 
         if ($session = $event->getRequest()->getSession()) {
-            $session->save();
+            if ($session->isStarted()) {
+                $session->save();
+            }
 
             $params = session_get_cookie_params();
 

src/Symfony/Bundle/FrameworkBundle/Tests/EventListener/TestSessionListenerTest.php

@@ -43,6 +43,7 @@ class TestSessionListenerTest extends \PHPUnit_Framework_TestCase
 
     public function testShouldSaveMasterRequestSession()
     {
+        $this->sessionHasBeenStarted();
         $this->sessionMustBeSaved();
 
         $this->filterResponse(new Request());
@@ -66,6 +67,14 @@ class TestSessionListenerTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals(0, reset($cookies)->getExpiresTime());
     }
 
+    public function testUnstartedSessionIsNotSave()
+    {
+        $this->sessionHasNotBeenStarted();
+        $this->sessionMustNotBeSaved();
+
+        $this->filterResponse(new Request());
+    }
+
     private function filterResponse(Request $request, $type = HttpKernelInterface::MASTER_REQUEST)
     {
         $request->setSession($this->session);
@@ -92,6 +101,20 @@ class TestSessionListenerTest extends \PHPUnit_Framework_TestCase
             ->method('save');
     }
 
+    private function sessionHasBeenStarted()
+    {
+        $this->session->expects($this->once())
+            ->method('isStarted')
+            ->will($this->returnValue(true));
+    }
+
+    private function sessionHasNotBeenStarted()
+    {
+        $this->session->expects($this->once())
+            ->method('isStarted')
+            ->will($this->returnValue(false));
+    }
+
     private function getSession()
     {
         $mock = $this->getMockBuilder('Symfony\Component\HttpFoundation\Session\Session')