Fixed error when params passed into new

Trying to pass parameters into a model's `new` action will currently fail if that model has any protected attributes.

This issue can be easily reproduced by visiting a link of the form `http://localhost:3000/rails_admin/some_model/new?some_model%5Bfoo%5D=bar` where `some_model` has protected attributes.

The previous implementation was trying to set the attributes using `@object.attributes.merge(object_params)`. Merging the params into the existing attributes isn't necessary because this is handled by `assign_attributes` anyway.


Any feedback welcomed!