Respond to CoreOS requests with the same body
    
    Responding with the `build_id` and `stream` is good, though let's just
    simplify and respond with the full message body.
    
    One use case for this is that I'd like the request side of this
    transaction (the FCOS pipeline) to just include a generated `request_id`
    to have a trivial foolproof way to match up requests and responses,
    instead of checking for `build_id` and `stream`.
    
    We could later on teach this `request_id` concept to RoboSignatory so
    we can have idempotence from a client somehow sending the same request
    multiple times.
    
    Signed-off-by: Jonathan Lebon <jonathan@jlebon.com>