FormPreserve fixes

This fixes some issues I reported in https://github.com/aldeed/meteor-autoform/pull/67. `getFormValues()` doesn't add the doc `_id` so when replacing the doc with the cache it's removed. Note these don't fix this for subforms, but that's not really an issue for me. I can't find where `_id` is set in autoform. Perhaps it's provided from the template. Perhaps have an option to disable FormPreserve - at least for me it's only been a headache fixing issues and malformed data and I don't care about form data resetting between migrations.
Also I think we would need to do `context.doc = _.extend(context.doc || {}, retrievedDoc);` since the doc may be empty when this is an insert form?