Faça stub em qualquer instância com miniteste

def stub_any_instance(klass, method, value)
  klass.class_eval do
    alias_method :"new_#{method}", method

    define_method(method) do
      if value.respond_to?(:call)
        value.call
      else
        value
      end
    end
  end

  yield
ensure
  klass.class_eval do
    undef_method method
    alias_method method, :"new_#{method}"
    undef_method :"new_#{method}"
  end
end