[BUGFIX] Don't pass function UNDEFINED as oldValue to computed properties

was: "Failing test if computed property receives function UNDEFINED as oldValue"
Hey,

currently if you get a computed property that is undefined (unknown?) and then set it, the computed property gets function UNDEFINED passed as oldValue

Demo of the issue:
http://emberjs.jsbin.com/vijajora/2/edit

Best regards,
Domme

P.S.: @stefanpenner this is likely for you :+1: