SetFromTest: add new test for not overruling existing Sender
SetFromTest: add new test for not overruling existing Sender

Based on the code in the method, any existing, previously set `Sender` should not be overruled, even when the `$auto` parameter is set to `true`.

This method tests that specific situation.

jimjag/PHPMailer,nawawi/PHPMailer,PHPMailer/PHPMailer,PHPMailer/PHPMailer,nawawi/PHPMailer,jimjag/PHPMailer