Bug 1062728 - [email/pop3] POP3 sync breaks on messages that lack a Date header. v2.0-specific uplift. r=mcav

This is a minimal v2.0 specific uplift of the trunk fix for bug 1062728.  It
only includes the date logic to fix the specific error.
Merge pull request #24556 from asutherland/email-v20-pop3-no-date

Bug 1062728 - [email/pop3] POP3 sync breaks on messages that lack a Date header. v2.0-specific uplift. r=mcav