Issues with bitmasker

Hi!

I'm trying your gem and I'm having issues setting the bitmasks correctly. I set my 7 bitmask_attributes for my model and I am seeing that, after setting the first one, the following to be set up will made the mask forget about the previous setting. 

I was playing with tests and I think I found some issues. I provide a failing test. Actually, the test that was designed to show my trouble is passing, but the ``test_default`` behaves weirdly. As a sidenote, If I set ``:with_false_default`` to ``0b01000``, everything will go green again. 

Does it ring any bell? 

Thanks !!