db/redis: FieldMap() function should support  the mapping of array of structs

We should support [mapping array of structs](https://github.com/NYTimes/video-transcoding-api/blob/master/db/redis/storage/redis.go#L99-L106) so we can correctly save [Job.Outputs](https://github.com/NYTimes/video-transcoding-api/blob/master/db/types.go#L48).