BUG: getPartWithContentId is tolowercase but not cid

[Tzanou123]

Hello,

I think i found a fix for a bug i get on showing inline attachments.

On https://github.com/Enough-Software/enough_mail/blob/main/lib/src/mime_message.dart

Line 1012 you check if partCid.toLowerCase() == cid but cid it is not in toLowerCase so i get a return null even if the part exist.

But Line 1008 the variable contentId = contentId.toLowerCase(); but not used

You maybe need to use it on line 1012 as follow partCid.toLowerCase() == contentId