object: Return undefined and not the actual function on disposed objects

When calling a proto function on a disposed object we return true not to throw, however when doing this we implicitly return the actual underlying function pointer and that may cause use the return value to be used to wrongly set a variable or to be wrongly evaluated.

To avoid this and be consistent, return undefined instead.

Adapt tests for this and add more for uncovered methods.

Fixes #396 (closed)