Change default domain from "ssp" to "messages".

lib/SimpleSAML/Locale/Localization.php

@@ -25,7 +25,7 @@ class Localization
     /**
      * The default gettext domain.
      */
-    const DEFAULT_DOMAIN = 'ssp';
+    const DEFAULT_DOMAIN = 'messages';
 
     /**
      * Old internationalization backend included in SimpleSAMLphp.

tests/lib/SimpleSAML/Locale/LocalizationTest.php

@@ -19,7 +19,7 @@ class LocalizationTest extends \PHPUnit_Framework_TestCase
         );
         $l = new Localization($c);
         $this->assertTrue($l->isI18NBackendDefault());
-        $this->assertEquals(Localization::DEFAULT_DOMAIN, 'ssp');
+        $this->assertEquals(Localization::DEFAULT_DOMAIN, 'messages');
         $this->assertEquals($l->getCurrentDomain(), Localization::DEFAULT_DOMAIN);
     }