Assign params in ResourceController#create instead of ResourceController#build_resource

ResourceController’s `#create` and `#update` methods differ in the way attributes are assigned to objects:

With `#update`, params are not assigned until `update_attributes(params[object_name])`is called. Before that the `update.before`callback is executed.

`#create` is different. Due to the way the `build_resource` method is defined, parameters are already assigned before the `#create` method is called. This also means, that attributes are already assigned when the `create.before` callback is executed.
This prevents users from filtering the `params`hash using controller callbacks.

Both `#create` and `#update` should be built the same way.