Previously, a composite primary key model would need to specify
either a `primary_key` or `query_constraints` option on its associations.
Without it, a `CompositePrimaryKeyMismatchError` would be raised.

Most of the time, the composite primary key includes the `:id` column,
and associations already expect that `:id` is being used as the
primary key for the association (and have a corresponding foreign_key
column set on the associated model).

Rather than requiring users to define `primary_key: :id` throughout
an application with composite primary key associations, we can infer
that the `:id` column should be used. Users can still override this
behaviour by specifying a `primary_key:` or `query_constraints:` on
the association.

Note that, if the composite primary key for a model does _not_ include
`:id`, Rails won't infer the primary key for any related associations,
and users must still specify `query_constraints` or `primary_key`.

Prior to this change, you'd need to do the following to set up
associations for composite primary key models:

```ruby
class Order
  self.primary_key = [:shop_id, :id]

  has_many :order_agreements, primary_key :id
end

class OrderAgreement
  belongs_to :order, primary_key: :id
end
```

After this change, the `primary_key` option no longer needs to be
specified:

```ruby
class Order
  self.primary_key = [:shop_id, :id]

  has_many :order_agreements
end

class OrderAgreement
  belongs_to :order
end
```