EncodeStringTest: add additional test cases
EncodeStringTest: add additional test cases

... which should be handled correctly based on the code in the method under test.

With these additional test cases, the method now has 100% code coverage and is fully tested.

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