Fix layout rendering issue when layout is explicitly set to false

If layout is explicitly set to false, and a template file is put in the outside directory with the same name as the directory itself (but with a template extension, so the real file names don't conflict), that file is used as layout.

A test case is added to duplicate the behavior. In the test case, a `false.jade` is put beside the directory `false`, which contains a `layout-false.jade` with layout set to false in `_data.json`. Without the patch, the `false.jade` file is used as the layout for `layout-false.jade`.

The fix is simple, explicitly adding a check for `layout === false` case when looking up layouts.