CreatedAt and UpdatedAt in API responses should be valid timstamps

[danail-branekov]

Background

Currently, created_at and updated_at fields from API reponse objects are plain strings (for example, the app response). While that is fine for the CF cli, programmatic clients, such as go-cfclient, do not accept plain strings

Acceptance

GIVEN I have pushed some apps
WHEN I list apps via

curl "https://api.example.org/v3/apps" \
  -X GET \
  -H "Authorization: bearer [token]"

THEN I get a reponse with valid created_at and updated_at timestamps

HTTP/1.1 200 OK
Content-Type: application/json

  {
    "pagination": {
      ...
    },
    "resources": [
      {
        "guid": "1cb006ee-fb05-47e1-b541-c34179ddc446",
        "name": "my_app",
        "state": "STARTED",
        "created_at": "2016-03-17T21:41:30Z",
        "updated_at": "2016-03-18T11:32:30Z",
        ...
      },
      ...
    ]
  }

Dev notes

• We are currently encoding the response body with a JSON encoder that has its escapeHTML option enabled. While this is convinient to escape special characters in the resource links, we believe that that escaping is messing up with timestamps
• The easiest way to solve this seems to simply change the type of CreatedAt and UpdatedAt fields to time.Time
• We need to apply that pattern to all the response types