merged branch ruimarinho/http_digest (PR #3778)

Commits
-------

fd1ea69 [Security] [HttpDigest] Fixes a configuration error caused by an invalid key child node configuration

Discussion
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[Security] [HttpDigest] Fixes a configuration error caused by an invalid "key" child node configuration

```
Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
```

src/Symfony/Bundle/SecurityBundle/DependencyInjection/Security/Factory/HttpDigestFactory.php

@@ -62,7 +62,7 @@ class HttpDigestFactory implements SecurityFactoryInterface
             ->children()
                 ->scalarNode('provider')->end()
                 ->scalarNode('realm')->defaultValue('Secured Area')->end()
-                ->scalarNode('key')->cannotBeEmpty()->end()
+                ->scalarNode('key')->isRequired()->cannotBeEmpty()->end()
             ->end()
         ;
     }