Fix export for yes_no champs

app/models/champ.rb

@@ -78,7 +78,7 @@ class Champ < ActiveRecord::Base
       when 'textarea'
         ActionView::Base.full_sanitizer.sanitize(value)
       when 'yes_no'
-        value == 'yes' ? 'oui' : 'non'
+        value == 'true' ? 'oui' : 'non'
       when 'multiple_drop_down_list'
         drop_down_list.selected_options_without_decorator(self).join(', ')
       else

spec/models/champ_spec.rb

@@ -88,16 +88,22 @@ describe Champ do
       let(:type_champ) { 'yes_no' }
 
       context 'if yes' do
-        let(:value) { 'yes' }
+        let(:value) { 'true' }
 
         it { expect(champ.for_export).to eq('oui') }
       end
 
       context 'if no' do
-        let(:value) { 'no' }
+        let(:value) { 'false' }
 
         it { expect(champ.for_export).to eq('non') }
       end
+
+      context 'if nil' do
+        let(:value) { nil }
+
+        it { expect(champ.for_export).to eq(nil) }
+      end
     end
 
     context 'when type_de_champ is multiple_drop_down_list' do