message: fall-back to received for dateless

For the (rare) messages without a Date: header, attempt to guess a date
from the top (most-recent) Received: header. Not perfect, of course, but
better than nothing.

Requires re-indexing to apply to already indexed messages.

Should help for #1083.

lib/message/mu-message.cc

@@ -741,7 +741,10 @@ fill_document(Message::Private& priv)
 				doc.add(field.id, priv.ctime);
 			break;
 		case Field::Id::Date:
-			if (const auto& date{mime_msg.date()}; date) {
+			/* for the rare message without a Date: header, fall back
+			 * to the most recent Received: header */
+			if (const auto date{mime_msg.date().or_else([&]{
+					return mime_msg.received(); })}; date) {
 				doc.add(field.id, date->first);
 				doc.add(Field::Id::UtcOffset, date->second);
 			}