fix forced image conversion from L to RGB(A)
fix forced image conversion from L to RGB(A)

camptocamp/mapproxy,olt/mapproxy,olt/mapproxy,mapproxy/mapproxy,vrsource/mapproxy,vrsource/mapproxy,procrastinatio/mapproxy,procrastinatio/mapproxy,drnextgis/mapproxy,camptocamp/mapproxy,drnextgis/mapproxy,mapproxy/mapproxy