Merge branch '5.1' into 5.2

* 5.1: Use ::class keyword when possible

Author: fabpot

Test/TranslatorTest.php

@@ -161,7 +161,7 @@ public function testReturnMessageIfExactlyOneStandardRuleIsGiven()
  */
  public function testThrowExceptionIfMatchingMessageCannotBeFound($id, $number)
  {
- $this->expectException('InvalidArgumentException');
+ $this->expectException(\InvalidArgumentException::class);
  $translator = $this->getTranslator();
 
  $translator->trans($id, ['%count%' => $number]);