post_test.rb: add more metatag search tests.

app/models/note.rb

@@ -94,7 +94,7 @@ class Note < ActiveRecord::Base
   end
 
   def initialize_creator
-    self.creator_id = CurrentUser.id
+    self.creator_id ||= CurrentUser.id
   end
 
   def initialize_updater

app/models/post.rb

@@ -173,7 +173,7 @@ class Post < ActiveRecord::Base
     end
     
     def is_animated_png?
-      if file_ext =~ /png/i
+      if file_ext =~ /png/i && File.exists?(file_path)
         apng = APNGInspector.new(file_path)
         apng.inspect!
         return apng.animated?