champ: return the parent dossier even when discarded

Dossier has a `default_scope { kept }`.

Because of that, when the parent dossier is discarded, `champ.dossier`
will return nil.

We should kill the default scope. But meanwhile, ensure that
`champ.dossier` returns even a discarded dossier.

spec/models/champ_spec.rb

@@ -3,6 +3,17 @@ describe Champ do
 
   it_should_behave_like "champ_spec"
 
+  describe "associations" do
+    it { is_expected.to belong_to(:dossier) }
+
+    context 'when the parent dossier is discarded' do
+      let(:discarded_dossier) { create(:dossier, :discarded) }
+      subject(:champ) { discarded_dossier.champs.first }
+
+      it { expect(champ.reload.dossier).to eq discarded_dossier }
+    end
+  end
+
   describe "validations" do
     let(:row) { 1 }
     let(:champ) { create(:champ, type_de_champ: create(:type_de_champ), row: row) }