Fix messages stuck in pending state

The unset function, and the series of events/callbacks triggered by its
use, are not as similar to the set/save functions as previously
anticipated, leading to flux in the state of the 'pending' attribute.

Fixes #283

// FREEBIE

js/models/conversations.js

@@ -97,8 +97,7 @@
             sendFunc = textsecure.messaging.sendMessageToGroup;
         }
         sendFunc(this.get('id'), body, attachments, now).then(function() {
-            message.unset('pending');
-            message.save();
+            message.save({'pending': false});
         }.bind(this)).catch(function(errors) {
             if (errors instanceof Error) {
                 errors = [errors];