bug symfony#25848 [Validator] add missing parent isset and add test (Simperfit)

This PR was merged into the 2.7 branch.

Discussion
----------

[Validator] add missing parent isset and add test

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | symfony#25843 <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        |

This is a little PR to fix the error and to add the parent::__isset to the Constraints class, I'm adding a test too to be sure that this is not possible again.

Btw, since this is instagihub, I'm coding at home and then I see a squirrel :
![img_3187 heic](https://user-images.githubusercontent.com/3451634/35149517-a9ddc0ba-fd16-11e7-9e22-cdba0b677071.jpeg)

Commits
-------

bcb79a3 [Validator] add missing parent isset and add test

Author: nicolas-grekas

src/Symfony/Component/Validator/Constraint.php

@@ -214,6 +214,16 @@ public function __get($option)
         throw new InvalidOptionsException(sprintf('The option "%s" does not exist in constraint %s', $option, get_class($this)), array($option));
     }
 
+    /**
+     * @param string $option The option name
+     *
+     * @return bool
+     */
+    public function __isset($option)
+    {
+        return 'groups' === $option;
+    }
+
     /**
      * Adds the given group if this constraint is in the Default group.
      *

src/Symfony/Component/Validator/Tests/Constraints/FileTest.php

@@ -26,6 +26,16 @@ public function testMaxSize($maxSize, $bytes, $binaryFormat)
 
         $this->assertSame($bytes, $file->maxSize);
         $this->assertSame($binaryFormat, $file->binaryFormat);
+        $this->assertTrue($file->__isset('maxSize'));
+    }
+
+    public function testMagicIsset()
+    {
+        $file = new File(array('maxSize' => 1));
+
+        $this->assertTrue($file->__isset('maxSize'));
+        $this->assertTrue($file->__isset('groups'));
+        $this->assertFalse($file->__isset('toto'));
     }
 
     /**