The `read_attribute` method always returns the primary key when asked to
read the `id` attribute, even if the primary key isn't named `id`, and
even if another attribute named `id` exists.

For the `inspect`, `attribute_for_inspect` and `pretty_print` methods,
this behaviour is undesirable, as they're used to examine the internal
state of the record. By using `_read_attribute` instead, we'll get the
real value of the `id` attribute.