Remove `X-` prefix from HTTP headers

[QuintinWillison]

We've introduced the Ably-Agent header (#1034, ably/docs#1077, ably/docs#1096) which (quite rightly) does not have an X- prefix, given it's been deprecated (see RFC 6648).

Work on this issue will include:

• [ ] Rename X-Ably-Version to Ably-Version (RSC7a)
• [ ] Remove X-Ably-Lib (RSC7b) - superseded by Ably-Agent (RSC7d2)
• [ ] Rename X-Ably-ClientId to Ably-ClientId (RSC7e2)
• [ ] Rename X-Ably-DeviceIdentityToken (RSH6a) to Ably-DeviceIdentityToken
• [ ] Rename X-Ably-DeviceSecret (RSH6b) to Ably-DeviceSecret
• [ ] Rename X-Ably-Errorcode (HP6) to Ably-ErrorCode
• [ ] Rename X-Ably-Errormessage (HP7) to Ably-ErrorMessage
• [ ] Checking for other X--prefixed headers in client-library codebases
• [ ] Checking for other X--prefixed headers in the Realtime (internal) codebase
• [ ] Creating ticket(s) for the Realtime team (internal code modifications)
• [ ] Creating tickets for the SDK Team client library repositories (open source code modifications)
• [ ] Confirming whether this work should be tagged under a new protocol version

See also this internal Slack thread, raised by @mattheworiordan.

┆Issue is synchronized with this Jira Task by Unito

[mattheworiordan]

@QuintinWillison it is not clear in this issue if we have introduced an inconsistency problem with the new header ably-agent or if we are going to keep x- for now for consistency and address this all in one batch. Would you mind clarifying?

[paddybyers]

All new headers will not include x-. There's no point adding it, and making this new entry wrong, just for the sake of consistency with all the other things that are also wrong. It would also create a new migration/interop requirement.

[sync-by-unito[bot]]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2772