Remove redundancy in _update fields
Remove redundancy in _update fields

Also, rename __get_opts_fields -> __filter_fields

0xDCA/marshmallow,etataurov/marshmallow,bartaelterman/marshmallow,Bachmann1234/marshmallow,0xDCA/marshmallow,jmcarp/marshmallow,daniloakamine/marshmallow,marshmallow-code/marshmallow,quxiaolong1504/marshmallow,xLegoz/marshmallow,jmcarp/marshmallow,dwieeb/marshmallow,VladimirPal/marshmallow,mwstobo/marshmallow,maximkulkin/marshmallow,Tim-Erwin/marshmallow