Merge branch '5.1'

* 5.1:
  [ErrorHandler] Return false directly and remove unused variable
  [OptionsResolver] Assert that the error type is valid in deprecations test
  [OptionsResolver] Fix deprecation message access
  [HttpClient] Allow bearer token with colon
  [Form] Fix custom formats deprecation with HTML5 widgets
  [Translator] Optional Intl dependency
  [Contracts][Translation] Optional Intl dependency
  [ErrorHandler] Escape JSON encoded log context
  update missing translations arabic
  [Yaml] simplify the test
  fix test by letting mock throw the actual expected exception

Author: fabpot

TranslatorTrait.php

@@ -35,7 +35,7 @@ public function setLocale(string $locale)
      */
     public function getLocale()
     {
-        return $this->locale ?: \Locale::getDefault();
+        return $this->locale ?: (class_exists(\Locale::class) ? \Locale::getDefault() : 'en');
     }
 
     /**