update bert.js to encode *undefined* value to the *undefined* atom
update bert.js to encode *undefined* value to the *undefined* atom

This simplifies exchanging nil values between the viewport and the
server. I picked *undefined* because it is already used in erlang.

nitrogen/nitrogen_core,choptastic/nitrogen_core,choptastic/nitrogen_core,choptastic/nitrogen_core,nitrogen/nitrogen_core,nitrogen/nitrogen_core,nitrogen/nitrogen_core