fix(mail(dav)): add support for property {DAV:}getcontentlength

SoObjects/Mailer/SOGoMailFolder.m

@@ -1776,6 +1776,8 @@ static NSInteger _compareFetchResultsByUID (id entry1, id entry2, NSArray *uids)
                              forKey: @"{urn:schemas:mailheader:}references"];
       [davIMAPFieldsTable setObject: @"BODY[HEADER.FIELDS (SUBJECT)]"
                              forKey: @"{DAV:}displayname"];
+      [davIMAPFieldsTable setObject: @"RFC822.SIZE"
+                             forKey: @"{DAV:}getcontentlength"];
       [davIMAPFieldsTable setObject: @"BODY[HEADER.FIELDS (TO)]"
                              forKey: @"{urn:schemas:mailheader:}to"];
     }

SoObjects/Mailer/SOGoMailObject.m

@@ -1273,7 +1273,7 @@ static BOOL debugSoParts       = NO;
 
 - (NSString *) davContentLength
 {
-  return [[self fetchCoreInfos] valueForKey: @"size"];
+  return [NSString stringWithFormat: @"%@", [[self fetchCoreInfos] valueForKey: @"size"]];
 }
 
 - (NSDate *) davCreationDate