Reading from undefined values in scope.

[justinbmeyer]

Related to: #762, if scope reads from an undefined value, it errors:

var scope = new can.view.Scope(undefined);

var compute = scope.compute("property");

equal(compute(), undefined, "got back undefined");

[matthewp]

I previously reported this and included a test: #1297

[daffl]

#1297 and this are two separate issues actually.