Merge branch '2.8' into 3.4

* 2.8:
  [Form] Hardened test suite for empty data
  Bump phpunit XSD version to 5.2
  Add required key attribute

Author: nicolas-grekas

phpunit.xml.dist

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/4.1/phpunit.xsd"
+         xsi:noNamespaceSchemaLocation="http://schema.phpunit.de/5.2/phpunit.xsd"
          backupGlobals="false"
          colors="true"
          bootstrap="vendor/autoload.php"