JSON API Extras @ResourceFieldEnhancer plugins should also be applied to filter values

Timestamp fields like "updatedAt" are represented in the JSONAPI attributes in ISO 8601 format. But you cannot echo this same format back into a filter value.

For example, given the content:

{
            "type": "recipes",
            "id": "455f52e3-212e-4af7-b46a-55d8b93229a7",
            "attributes": {
                 ...
                "updatedAt": "2017-07-22T09:00:14+0200"
                 ...
             }
}

this filter (which represents updatedAt < 2017-07-23T00:00:00+0200) does not match:

http://live-contentacms.pantheonsite.io/api/recipes?filter%5BupdatedAt%5D%5Bvalue%5D=2017-07-23T00%3A00%3A00%200200&filter%5BupdatedAt%5D%5Boperator%5D=%3C

Whereas if I convert the date to a unix timestamp, it matches:

http://live-contentacms.pantheonsite.io/api/recipes?filter%5BupdatedAt%5D%5Bvalue%5D=1500760800&filter%5BupdatedAt%5D%5Boperator%5D=%3C

For consistency, I suspect filter values should pass through the same format conversion that is used for attributes during a POST or PATCH.