String values in mapping cause infinite recursion in extendObject

[lawnsea]

Repro:
Run the following in the console of the unit tests:

    var model = ko.mapping.fromJS({ foo: 'bar' }, {
        fiz: 'applesauce'
    });

    ko.mapping.fromJS({ foo: 'baz' }, model);

Expected:
Model.foo -> 'baz'

Observed:
extendObject recurses infinitely on 'a' (because 'a'[0] == 'a').

We ran into this after wrapping fromJS to support other mapping options, some of which have string values. I realize that there are no mapping options for which a string is a legal value in vanilla knockout, but this is a particularly gnarly way to crash, as you lose the stack trace and debuggers don't break on the error.

We have to fix this for our project, so expect a pull request shortly.