Fix User model validation behavior, closes #950

tests/Unit/Rules/UsernameTest.php

@@ -7,6 +7,14 @@ use Pterodactyl\Rules\Username;
 
 class UsernameTest extends TestCase
 {
+    /**
+     * Test that this rule can be cast to a string correctly.
+     */
+    public function testRuleIsStringable()
+    {
+        $this->assertSame('p_username', (string) new Username);
+    }
+
     /**
      * Test valid usernames.
      *