change default string crypter to AESStringCrypter

Pilz.Cryptography/SecureString.cs

@@ -50,7 +50,7 @@ namespace Pilz.Cryptography
             if (Crypter == null)
             {
                 if (DefaultCrypter == null)
-                    DefaultCrypter = new SimpleStringCrypter(string.Empty);
+                    DefaultCrypter = new AESStringCrypter();
                 Crypter = DefaultCrypter;
             }
             return Crypter;