minor silexphp#1638 Simplify extend call (hkdobrev)

This PR was merged into the 2.3.x-dev branch.

Discussion
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Simplify extend call

Since Pimple 2.x there is no need to re-assign the result from `extend()` call to the same key. Pimple already does that.

Commits
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84bcb9b Simplify extend call

tests/Silex/Tests/Provider/ValidatorServiceProviderTest.php

@@ -155,7 +155,7 @@ public function testAddResource($registerValidatorFirst)
 
         $app->register(new ValidatorServiceProvider());
         $app->register(new TranslationServiceProvider());
-        $app['translator'] = $app->extend('translator', function ($translator, $app) {
+        $app->extend('translator', function ($translator, $app) {
             $translator->addResource('array', ['This value should not be blank.' => 'Pas vide'], 'fr', 'validators');
 
             return $translator;