update foreign id with remote post

WordPress/Classes/Services/PostHelper.m

@@ -92,6 +92,7 @@ + (void)updatePost:(AbstractPost *)post withRemotePost:(RemotePost *)remotePost
             publicizeMessage = [publicizeMessageDictionary stringForKey:@"value"];
             publicizeMessageID = [publicizeMessageDictionary stringForKey:@"id"];
         }
+        postPost.foreignID = remotePost.foreignID;
         postPost.publicID = publicID;
         postPost.publicizeMessage = publicizeMessage;
         postPost.publicizeMessageID = publicizeMessageID;