eyre: look for lowercase last-event-id header
Eyre always gets passed request headers in lowercase, so we should search for
the lowercased version of the header.
Arguably `+get-header` should lowercase keys before comparing them, but that's
a more serious behavioral change.
:: the request may include a 'Last-Event-Id' header
::
=/ maybe-last-event-id=(unit @ud)
- ?~ maybe-raw-header=(get-header:http 'Last-Event-ID' header-list.request)
+ ?~ maybe-raw-header=(get-header:http 'last-event-id' header-list.request)
~
(rush u.maybe-raw-header dum:ag)
:: flush events older than the passed in 'Last-Event-ID'
