Ensure that decode returns all parts of composite key

[plabaj]

This pull request addresses a bug in the decoding of composite keys. In the current implementation, when a model is defined with a composite primary key, the decoding process incorrectly strips out the first key component. This fix ensures that all components of the composite key are retained and correctly decoded.

For models with composite keys, such as the following example:

class Product < ApplicationRecord
  has_prefix_id :prd
  self.primary_key = [:store_id, :sku]
end

The decode_prefix_id method fails to decode the composite key correctly. Specifically, when decoding the prefix_id for a Product instance, the resulting composite key array incorrectly omits the first key component.

Example to illustrate the issue:

p = Product.create(store_id: 1, sku: 10)
composite = Product.decode_prefix_id(p.prefix_id)

Currently, the composite array is:

[10]

However, the expected result should be:

[1, 10]

This fix ensures that the decode_prefix_id method properly handles composite keys, preserving all components as intended.

[excid3]

Thanks @plabaj 👍