CYB-340 Comment on own gallery item should NOT score points for giving or receiving

* Comparison was being made between different data types; the poster's id on the parent
   item was an integer (Fixnum) but the current_user.canvas_id was a String.
  Calling '#to_s' on both, if later current_user.canvas_id is refactored to return an Integer,
   this will make sure that even if this code is missed, it will still work correctly.  #to_s
   on a String just returns self.

https://jira.ets.berkeley.edu/jira/browse/CYB-340
Merge pull request #232 from coyote/compare_like_types_in_comments_controller

Comment on own gallery item should NOT score points for giving or receiving