Initialize instance with non-prefix attributes in ActiveRecord::Base.build

`ActiveResource::Base.build` used to use the attributes to initialize the instance. That functionality was lost when the attributes started to be passed to `new_element_path` (see https://github.com/rails/activeresource/commit/fd3616a46b90b10015a47b1275c1109ab836e9f3).

This PR re-adds the functionality.