Fix a bug when subclassing serializer
Fix a bug when subclassing serializer

bodylabs/drf-to-s3,bodylabs/drf-to-s3,pombredanne/drf-to-s3,pombredanne/drf-to-s3,bodylabs/drf-to-s3,pombredanne/drf-to-s3,pombredanne/drf-to-s3,bodylabs/drf-to-s3,pombredanne/drf-to-s3,bodylabs/drf-to-s3