dossiers: render JSON if needed

When receiving a request that expects JSON, return a simple '200'.

This avoids the unecessary work of rendering all the HTML page (which
ultimately will not be used).

app/controllers/users/dossiers_controller.rb

@@ -143,18 +143,19 @@ module Users
 
       errors = update_dossier_and_compute_errors
 
-      if errors.present?
-        flash.now.alert = errors
-        render :brouillon
-      else
-        if passage_en_construction?
+      if passage_en_construction? && errors.blank?
         @dossier.en_construction!
         NotificationMailer.send_initiated_notification(@dossier).deliver_later
-          redirect_to merci_dossier_path(@dossier)
+        return redirect_to(merci_dossier_path(@dossier))
+      elsif errors.present?
+        flash.now.alert = errors
       else
         flash.now.notice = 'Votre brouillon a bien été sauvegardé.'
-          render :brouillon
       end
+
+      respond_to do |format|
+        format.html { render :brouillon }
+        format.json { head :ok }
       end
     end