Fixes identity property and adds readonly property.
Fixes identity property and adds readonly property.

This is also to sync with current state of Heroku's Platform API
as well as to make sure it follows the HTTP API Design Guide.

uberjay/prmd,robhurring/prmd,niaeashes/prmd,IceDragon200/prmd,hx/prmd,junya-i/prmd,michaelsauter/prmd,shopkeep/prmd,IceDragon200/prmd,interagent/prmd,uberjay/prmd,michaelsauter/prmd,niaeashes/prmd,ruprict/prmd,MobileDevHQ/prmd,hmac/prmd,ruprict/prmd,hx/prmd,shopkeep/prmd,robhurring/prmd,araines/prmd,mlr/prmd,hmac/prmd,mlr/prmd,MobileDevHQ/prmd,junya-i/prmd,araines/prmd,interagent/prmd