check if resolutions is a list before attempt to sort
check if resolutions is a list before attempt to sort

--HG--
branch : 0.8.x

Anderson0026/mapproxy,olt/mapproxy,Anderson0026/mapproxy,camptocamp/mapproxy,kaiCu/mapproxy,kaiCu/mapproxy,faegi/mapproxy,faegi/mapproxy,drnextgis/mapproxy,mapproxy/mapproxy,camptocamp/mapproxy,geoadmin/mapproxy,procrastinatio/mapproxy,vrsource/mapproxy,geoadmin/mapproxy,vrsource/mapproxy,procrastinatio/mapproxy,mapproxy/mapproxy,drnextgis/mapproxy,olt/mapproxy