Fixed an issue: ko.isWriteableObservable incorrectly returns false for computed observables in the mapping context.

The withProxyDependentObservable function replaces ko.dependentObservable, but does not replace ko.isWriteableObservable, which causes the latter to always return false for computeds. 
At the same time, the mapping plugin itself uses isWriteableObservable a lot in the mapping process to determine whether it should use the existing observable or replace it with a new, writeable one. Since all computeds look like non-writeable at this point, the mapping procedure will replace all computeds with new observables, thus screwing up the viewmodel inner mechanics.

The solution is simple: when replacing ko.dependentObservable with a proxy, also replace ko.isWriteableObservable.