fix stream item generation for messages

Change-Id: I0f7deff680e80127d06fe4336d577cad5545689a
Reviewed-on: https://gerrit.instructure.com/126455
Reviewed-by: Dan Minkevitch <dan@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

app/models/stream_item.rb

@@ -153,7 +153,7 @@ class StreamItem < ActiveRecord::Base
 
   ROOT_DISCUSSION_ENTRY_LIMIT = 3
   def generate_data(object)
-    self.context ||= object.context rescue nil
+    self.context ||= object.try(:context) unless object.is_a?(Message)
 
     case object
     when DiscussionTopic