Fix defaultLayout issue...
Fix defaultLayout issue...

- Calling layout.template(false) or layout.template(null) would both end up
  giving you the default layout template. But since the values are different,
  the dep would be changed causing a re-render.
- Solution: in the layout.template(value) call check whether value is null or
  false and if so set the value to _defaultLayout.

cmather/blaze-layout