Merge branch 'fix-note-update' into 'master'

Fix note update

In some strange cases Ruby thinks `system` is not AR field but
Kernel.system call and raises exception during save. It should prevent
this random wierd behaviour.

Exception log:

```
ArgumentError (wrong number of arguments (0 for 1+)):
  app/models/note.rb:505:in `system'
  app/models/note.rb:505:in `editable?'
  app/controllers/projects/notes_controller.rb:33:in `update'
```

See merge request !1326

app/models/note.rb

@@ -502,6 +502,6 @@ class Note < ActiveRecord::Base
   end
 
   def editable?
-    !system
+    !read_attribute(:system)
   end
 end