add failing test when explicitly set layout to false in _data.json

When explicitly set layout to false in _data.json and put a file
in the outside directory with the same name as the directory name
and a template extension, that file will be used as layout. In the
test case, the file is false.jade outside the false/ directory.

This is because false is not explicitly checked in the layout
lookup, and is treated as empty string.
explicitly catch layout set to false case, and set layout to null
Merge pull request #13 from echaozh/fix-layout-set-to-false

Fix layout rendering issue when layout is explicitly set to false