fixes OIIO issue 518, extra_attribs properly responds to iterator calls
fixes OIIO issue 518, extra_attribs properly responds to iterator calls

cwilling/oiio,scott-wilson/oiio,OpenImageIO/oiio,bdeluca/oiio,bdeluca/oiio,micler/oiio,scott-wilson/oiio,micler/oiio,cwilling/oiio,mcanthony/oiio,cwilling/oiio,lgritz/oiio,sambler/oiio,OpenImageIO/oiio,YangYangTL/oiio,mcanthony/oiio,lgritz/oiio,bdeluca/oiio,OpenImageIO/oiio,YangYangTL/oiio,cwilling/oiio,sambler/oiio,mcanthony/oiio,mcanthony/oiio,lgritz/oiio,micler/oiio,sambler/oiio,bdeluca/oiio,YangYangTL/oiio,lgritz/oiio,scott-wilson/oiio,scott-wilson/oiio,sambler/oiio,OpenImageIO/oiio,YangYangTL/oiio,micler/oiio