Fix sync request argument check

Fixes #609

// FREEBIE

js/libtextsecure.js

@@ -37718,7 +37718,7 @@ textsecure.MessageSender.prototype = {
     window.textsecure = window.textsecure || {};
 
     function SyncRequest(sender, receiver) {
-        if (!(sender instanceof MessageSender) || !(receiver instanceof MessageReceiver)) {
+        if (!(sender instanceof textsecure.MessageSender) || !(receiver instanceof textsecure.MessageReceiver)) {
             throw new Error('Tried to construct a SyncRequest without MessageSender and MessageReceiver');
         }
         this.receiver = receiver;

libtextsecure/sync_request.js

@@ -8,7 +8,7 @@
     window.textsecure = window.textsecure || {};
 
     function SyncRequest(sender, receiver) {
-        if (!(sender instanceof MessageSender) || !(receiver instanceof MessageReceiver)) {
+        if (!(sender instanceof textsecure.MessageSender) || !(receiver instanceof textsecure.MessageReceiver)) {
             throw new Error('Tried to construct a SyncRequest without MessageSender and MessageReceiver');
         }
         this.receiver = receiver;