Simplify SMTP auth method selection

Instead of using the auth method name, let's just not supply username
and password when we don't want to authenticate. The two affected
servers AU and CA don't have credentials set anyway. This is compatible.

The specs needed changing though.

lib/spree/core/mail_settings.rb

@@ -20,18 +20,14 @@ module Spree
       private
 
       def mail_server_settings
-        settings = if need_authentication?
-                     basic_settings.merge(user_credentials)
-                   else
-                     basic_settings
-                   end
+        settings = basic_settings.merge(user_credentials)
 
         settings.merge(enable_starttls_auto: secure_connection?)
       end
 
       def user_credentials
-        { user_name: Config.smtp_username,
-          password: Config.smtp_password }
+        { user_name: Config.smtp_username.presence,
+          password: Config.smtp_password.presence }
       end
 
       def basic_settings
@@ -42,13 +38,10 @@ module Spree
       end
 
       def authentication
-        return unless need_authentication?
-
-        Config.mail_auth_type.presence
-      end
-
-      def need_authentication?
-        Config.mail_auth_type != 'None'
+        # "None" is an option in the UI but not a real authentication type.
+        # We should remove it from our host configurations but I'm keeping
+        # this check for backwards-compatibility for now.
+        Config.mail_auth_type.presence unless Config.mail_auth_type == "None"
       end
 
       def secure_connection?