[local-sync] Fix message parsing when no Date header present
[local-sync] Fix message parsing when no Date header present

Summary:
When the Date: header is not present, use the INTERNALDATE from the IMAP server
instead.

Test Plan: manual for now - will add a regression test for this though

Reviewers: juan, jackie

Reviewed By: jackie

Differential Revision: https://phab.nylas.com/D3556

simonft/nylas-mail,nylas/nylas-mail,simonft/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,nirmit/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,nylas-mail-lives/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,nylas/nylas-mail,nirmit/nylas-mail,nylas/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail