JSON dates should be ISO 8601 format strings

From mailing list:

> When I fetch something which has a Date member then the json contains something like 
> {
> "currentSeason": "N",
> "openStatus": "O",
> "seasonEndDate": new Date(1543536000000),
> "seasonId": 14,
> "seasonStartDate": new Date(1512086400000),
> },
> 

JSON dates should instead be formated to ISO 8601 strings such as:
`2012-04-23T18:25:43.511Z`