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.
Just thinking.... maybe it would be best to do `_.extend(context.doc, retrievedDoc)`? That way it will overwrite only the values that retrievedDoc has and keep any others (including _id).