add validation presence user for dossier

spec/decorators/dossier_decorator_spec.rb

@@ -1,7 +1,7 @@
 require 'spec_helper'
 
 describe DossierDecorator do
-  let(:dossier) { create(:dossier) }
+  let(:dossier) { create(:dossier, :with_user) }
   subject { dossier.decorate }
   describe 'last_update' do
     subject { Timecop.freeze(Time.new(2015, 12, 24, 14, 10)) { super().last_update } }