Don't merge: Bug with sa-defaults and auto_now

Bug occurs when used in conjunction with sqlalchemy-defaults, and was
introduced by ca119b08f.

Wasn't sure whether to report it here or to sqlalchemy-defaults, but since the bug was introduced here I though it might get fixed here as well. This was working with the latest version of sa-defaults and wtforms-alchemy 0.8.5, but broke on the update to 0.8.6 (the single commit mentioned above).

As I'm not really sure what's going on I haven't been able to find a fix that doesn't break all the other test cases, I hope you can come up with something, @kvesteri. Not sure if you want to include sa-defaults to run the test eventually, but it was the easiest way to replicate the issue.