Feeds for devices now return a readonly product_id and device_serial val

This was breaking the client as it wasn't looking for those keys; so
this commit adds these two fields into the _readonly_ fields on Feed,
with a corresponding test case.
auto_feed_url also present as read only field, so needs to be handled
Merge pull request #12 from smulube/new_fields

Feeds for devices now return a readonly product_id and device_serial val