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.
I was thinking that as well, but was worried about it not being a deep copy. We could use http://stackoverflow.com/questions/14843815/recursive-extend-in-underscore-js for that if necessary, but I think using `_.extend()` is as good as replacing the entire doc.