update Base64 trick, %z specifier in dates

[neimanpinchas]

I am trying to use this lib with haxe 4.3, I found that afew changes are needed in order to make it compatiable

#1 The %z time zone is no longer avilible so we must calculate it automatically (or remove the date header, my email server is capable of adding it manually
#2 haxe serializer, doesn't make anymore compatiable base64, so we could use it anymore to convert the attachments.

Do you recommand that I try to contribute these changes to this repo? or some other library is recommanded nowdays.

Thanks