fix #71
fix #71

fix https://github.com/incuna/django-wkhtmltopdf/issues/71
tclancy/django-wkhtmltopdf,incuna/django-wkhtmltopdf,halfnibble/django-wkhtmltopdf,tclancy/django-wkhtmltopdf,incuna/django-wkhtmltopdf,halfnibble/django-wkhtmltopdf