draft-ietf-jmap-core: X-JMAP-... header fields need to be renamed and registered with IANA

[aamelnikov]

5. Uploading binary data

There is a single endpoint which handles all file uploads, regardless
of what they are to be used for. To upload a file, the client
submits a POST request to the file upload endpoint (see the
authentication section for information on how to obtain this URL).
The Content-Type MUST be correctly set for the type of the file being
uploaded. The request MUST be authenticated as per any HTTP request.
The request MAY include an "X-JMAP-AccountId" header, with the value

Change the header field name not to use "X-" and register it in the IANA registry.

being the account to use for the request. Otherwise, the default
account will be used.