Conversation.sendMessage: Don't set unreadCount to zero

This is no longer guaranteed to be true. If you're scrolled up in a
conversation, you may not have read all messages. Setting the
unreadCount to zero will prevent the user from marking any of their
existing messages as unread until something else happens, like receiving
a read receipt or new message.

FREEBIE

js/models/conversations.js

@@ -165,7 +165,6 @@
             message.save();
 
             this.save({
-                unreadCount : 0,
                 active_at   : now,
                 timestamp   : now,
                 lastMessage : message.getNotificationText()