Stop clobbering conversation attributes

Unless the background page fetches the latest details of a conversation
before updating it, it may clobber or nullify some attributes e.g., the
contact's name.

js/background.js

@@ -93,6 +93,7 @@
             type : 'private'
         }, { merge : true } );
 
+        conversation.fetch().always(function() {
             var message = messages.add({
                 source         : pushMessage.source,
                 sourceDevice   : pushMessage.sourceDevice,
@@ -132,6 +133,7 @@
                     });
                 });
             });
+        });
     };
 
     extension.on('message:decrypted', function(options) {