Merge branch '2.8' into 3.1

* 2.8:
  [TwigBridge] removed deprecations added in Twig 1.27
  PHP CS Fixer: use php_unit_dedicate_assert
  3.0 Upgrade Guide: Added details describing how to pass data to a form through the options resolver
  fixed Filesystem:makePathRelative and added 2 more testcases
  no 304 response if method is not cacheable
  move tags from decorated to decorating service

Author: fabpot

Tests/Dumper/FileDumperTest.php

@@ -46,7 +46,7 @@ public function testDumpBackupsFileIfExisting()
         $dumper = new ConcreteFileDumper();
         $dumper->dump($catalogue, array('path' => $tempDir));
 
-        $this->assertTrue(file_exists($backupFile));
+        $this->assertFileExists($backupFile);
 
         @unlink($file);
         @unlink($backupFile);
@@ -65,7 +65,7 @@ public function testDumpCreatesNestedDirectoriesAndFile()
         $dumper->setRelativePathTemplate('test/translations/%domain%.%locale%.%extension%');
         $dumper->dump($catalogue, array('path' => $tempDir));
 
-        $this->assertTrue(file_exists($file));
+        $this->assertFileExists($file);
 
         @unlink($file);
         @rmdir($translationsDir);