Update invoice payer_email and description as optional

[wwchen21227]

https://github.com/xendit/xendit-php/blob/e4df989aac9e71512a544e7cb69f863d51adcfe3/src/Invoice.php#L49

[iprastha]

This resulted in inconsistencies between the documentation ( https://developers.xendit.co/api-reference/?php#create-invoice ) and the actual result, without parsing payer_email and description (marked as optional in the docs), invoice won't be created

[derryberni]

I also experienced the same thing, need fix it because make confused

[David-Kurniawan]

#147

[adityarx]

Closing as resolved~