Change instance-Id and deprecated-Id headers

[enketo-issue-mover]

From @MartijnR on July 20, 2016 22:20

It's not a great use of custom HTTP headers, and they are only used by Enketo internally (to log submissions), so they should at least be called "X-Enketo-...". If maintaining these headers is necessary, see if they can be stripped when the request is piped to the OpenRosa server.

Copied from original issue: kobotoolbox/enketo-express#536

[enketo-issue-mover]

From @MartijnR on July 20, 2016 22:27

These kind of practices could become problems when servers start relying on them.