Fallback to "Delivery-date" if message has no "Date"

b/10543163
Some servers don't put a "Date" header in the message.
If this happens, use "Delivery-date" for the date. At
least this way we don't get all messages seeming to come
from Dec 31 1969.

Change-Id: I6b2d41e65861bda6d844b4a672b8df17f6de8edf
1 file changed