Fix default procs' behavior when overriding keywords in subclasses

[maxim]

Portrayal relies on an ordered ruby hash to initialize keywords in the correct order. However, if overriding the same keyword in a subclass (by declaring it again), it didn't move keyword to the bottom of the hash, so this would happen:

class Person
  extend Portrayal
  keyword :email, default: nil
end

class Employee < Person
  keyword :employee_id
  keyword :email, default: proc { "#{employee_id}@example.com" }
end

employee = Employee.new(employee_id: '1234')
employee.email # => "@example.com"

The email is broken because it relies on having employee_id declared before email, but email was already declared first in the superclass. This change fixes situations like this by re-adding the keyword to the bottom of the hash on every re-declaration.

Additional commits in here:

• Update github rspec workflow with Ruby 3, and non-deprecated action
• Add readme section for subclassing

[cover]

Looks great 👍

[phillipgray]

Solid fix. 🚢