Allow date objects and other instances as values in nested objects.

Date objects are removed from objects when flattening. This check is the most reliable way to detect an object to be flattened.

    var values = {
      some: {
        nested: {
            key: 'foobar',
            date: new Date()
        }
      }
    };

    Dottie.flatten(values)
    > Object {some.nested.key: "foobar", some.nested.date: Thu Jun 19 2014 13:26:13 GMT+0200 (CEST)}
      some.nested.date: Thu Jun 19 2014 13:26:13 GMT+0200 (CEST)
      some.nested.key: "foobar"
      __proto__: Object