SetTest: split off value toggling test
SetTest: split off value toggling test

This commit moves the "set a property to null and then back" test to a separate test method.

Note: based on the code in the method, this test doesn't really add any value, but I also see no reason to remove it.

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