fixes resource mass-assignment to ignore private / unknown properties in lieu of exceptions

I was getting frustrated by the following unexpected behavior:

```
model = Model.new params[:model]
# => ArgumentError: The attribute _x_ is not accessible
```

The patch simply deletes this line: https://github.com/datamapper/dm-core/blob/master/lib/dm-core/resource.rb#L321

It's helpful to silence the errors, so I don't have to worry about the exact structure of the JSON getting thrown at the server. Instead, I can safely mass-assign and get the properties I want.