added tests for picker definitions including different combos of columns and extra_headers
added tests for picker definitions including different combos of columns and extra_headers

caktus/django-file-picker,Capstrat/django-file-picker,Capstrat/django-file-picker,caktus/django-file-picker,Capstrat/django-file-picker,Capstrat/django-file-picker,caktus/django-file-picker,caktus/django-file-picker