[#920] prefill siret when user has a siret

spec/features/france_connect/france_connect_spec.rb

@@ -72,6 +72,10 @@ feature 'France Connect Connexion' do
         scenario 'he is redirected to siret page' do
           expect(page).to have_css('#users_siret_index')
         end
+
+        scenario 'the siret is already written in form' do
+          expect(page.find_by_id('siret').value).to have_content(siret)
+        end
       end
     end
   end