Identity is defined by RFC 8621 under urn:ietf:params:jmap:submission, not
under mail. ihasmail asked for mail alone, so Stalwart 0.16 rejected both
Identity/get and Identity/set with unknownMethod: no identities were ever
listed, none could be created, and sending then failed with "No sending
identity available". Older Stalwart builds accepted the calls anyway, which
is why this went unnoticed.
Also filter `using` down to the capabilities the session actually advertises.
A server must reject the entire request with unknownCapability when `using`
names something it does not implement, so one over-eager urn would take down
every call sharing the batch — including, on a server predating the submission
capability, the mailbox and message loads batched alongside an identity fetch.
Fixes#12
