Setting dynamic attrs causes problem when mongo backed domain is using class inheritance.

Domains like `Certificate` and `SpecialCert extends Certificate`. Things work well. `_class` is added to each document when doing inserts.

However, since upgrade to grails 2.5.2 and 3.0.3 version of the mongo plugin I now have a problem when adding dynamic attributes before the first save. It causes the _class not to be inserted into mongo, and this in turn causes something like `SpecialCert.get(id)` to not return the document later.

There is a workaround. Just do the .save() first then add the props and do another .save(), but it is a definite gotcha.

``` groovy
if (!specialCert.hasErrors()) {
specialCert.save()
// for some reason if we try to stamp the markerKey before the first .save() GORM mongo
// loses that what class this is and misses putting the _class into the DB causing probs later
String markerKey = ourService.getMarkerKey()
if (markerKey) specialCert['markerKey'] = markerKey
specialCert.save()
}
```

Start by reproducing the shown sequence with a Certificate subclass, assigning a dynamic markerKey before the first save. Inspect the Grails Mongo persistence path for new domain instances and verify whether the inserted document contains _class; done means SpecialCert.get(id) returns the saved document without requiring a preliminary save.

Written by the indexing model from the issue text.