[geary] Fix (part 2): don't use time_t as property.

[Niels De Graef <nielsdg src gnome org>]

commit 756cc39d8de0f1b1340ba963274e5073b99515ba
Author: Niels De Graef <nielsdegraef gmail com>
Date:   Fri Sep 15 00:30:45 2017 +0200

    Fix (part 2): don't use time_t as property.
    
    See the previous commit message for the explanation.
    (this breaks builds in newer Vala versions)

 src/engine/imap-db/imap-db-message-row.vala |    2 +-
 src/engine/rfc822/rfc822-message-data.vala  |   21 +++++++++++++++------
 2 files changed, 16 insertions(+), 7 deletions(-)
---
diff --git a/src/engine/imap-db/imap-db-message-row.vala b/src/engine/imap-db/imap-db-message-row.vala
index 9d4b0d1..f9c294c 100644
--- a/src/engine/imap-db/imap-db-message-row.vala
+++ b/src/engine/imap-db/imap-db-message-row.vala
@@ -183,7 +183,7 @@ private class Geary.ImapDB.MessageRow {
         
         if (email.fields.is_all_set(Geary.Email.Field.DATE)) {
             date = (email.date != null) ? email.date.original : null;
-            date_time_t = (email.date != null) ? email.date.as_time_t : -1;
+            date_time_t = (email.date != null) ? email.date.to_time_t() : -1;
             
             fields = fields.set(Geary.Email.Field.DATE);
         }
diff --git a/src/engine/rfc822/rfc822-message-data.vala b/src/engine/rfc822/rfc822-message-data.vala
index 541ad10..44593f8 100644
--- a/src/engine/rfc822/rfc822-message-data.vala
+++ b/src/engine/rfc822/rfc822-message-data.vala
@@ -158,23 +158,25 @@ public class Geary.RFC822.MessageIDList : Geary.MessageData.AbstractMessageData,
 
 public class Geary.RFC822.Date : Geary.RFC822.MessageData, Geary.MessageData.AbstractMessageData,
     Gee.Hashable<Geary.RFC822.Date> {
+
+    private time_t as_time_t;
+
     public string? original { get; private set; }
     public DateTime value { get; private set; }
-    public time_t as_time_t { get; private set; }
     
     public Date(string iso8601) throws ImapError {
-        as_time_t = GMime.utils_header_decode_date(iso8601, null);
+        this.as_time_t = GMime.utils_header_decode_date(iso8601, null);
         if (as_time_t == 0)
             throw new ImapError.PARSE_ERROR("Unable to parse \"%s\": not ISO-8601 date", iso8601);
         
-        value = new DateTime.from_unix_local(as_time_t);
+        value = new DateTime.from_unix_local(this.as_time_t);
         original = iso8601;
     }
     
     public Date.from_date_time(DateTime datetime) {
         original = null;
         value = datetime;
-        as_time_t = Time.datetime_to_time_t(datetime);
+        this.as_time_t = Time.datetime_to_time_t(datetime);
     }
     
     /**
@@ -184,10 +186,17 @@ public class Geary.RFC822.Date : Geary.RFC822.MessageData, Geary.MessageData.Abs
         // Although GMime documents its conversion methods as requiring the tz offset in hours,
         // it appears the number is handed directly to the string (i.e. an offset of -7 becomes
         // "-0007", whereas we want "-0700").
-        return GMime.utils_header_format_date(as_time_t,
+        return GMime.utils_header_format_date(this.as_time_t,
             (int) (value.get_utc_offset() / TimeSpan.HOUR) * 100);
     }
-    
+
+    /**
+     * Returns {@link Date} as a time_t representation.
+     */
+    public time_t to_time_t() {
+        return this.as_time_t;
+    }
+
     /**
      * Returns {@link Date} for transmission.
      *