Only flatten validated attributes

When we flatten an entire attributes hash of a model, *lots* of things can go wrong. We can run into other models (through associations); we can run into a [file input's data object](http://blueimp.github.io/jQuery-File-Upload/); there's many more examples of things that will fail to be fully flattened.

Instead of trying to solve this piecemeal, here's a simple solution: Only flatten those attributes that we are validating. Those are the only ones we care about, anyhow. Problem solved.

Also see discussion in #125.