posts.rb: vote on behalf of correct user when moving favorites.

Bug: when an approver moves the favorites of a post, each favorite is
removed from the child post and added to the parent post. For gold+
users, this triggers an upvote, but these upvotes were performed by the
approver rather than the favoriter.

app/models/post_vote.rb

@@ -35,7 +35,7 @@ class PostVote < ActiveRecord::Base
   end
 
   def initialize_user
-    self.user_id = CurrentUser.user.id
+    self.user_id ||= CurrentUser.user.id
   end
 
   def update_post_on_destroy