Merge branch '2.7' into 2.8

* 2.7:
  prefer getSourceContext() over getSource()
  [HttpFoundation] Avoid implicit null to array conversion in request matcher
  Revert "bug #20184 [FrameworkBundle] Convert null prefix to an empty string in translation:update (chalasr)"
  Update UPGRADE-2.7.md

Author: fabpot

Fragment/HIncludeFragmentRenderer.php

@@ -140,12 +140,16 @@ private function templateExists($template)
         }
 
         $loader = $this->templating->getLoader();
-        if ($loader instanceof \Twig_ExistsLoaderInterface) {
+        if ($loader instanceof \Twig_ExistsLoaderInterface || method_exists($loader, 'exists')) {
             return $loader->exists($template);
         }
 
         try {
-            $loader->getSource($template);
+            if (method_exists($loader, 'getSourceContext')) {
+                $loader->getSourceContext($template);
+            } else {
+                $loader->getSource($template);
+            }
 
             return true;
         } catch (\Twig_Error_Loader $e) {