An event with an empty id field must reset the last event ID, so that
no Last-Event-ID header is sent on reconnect. run_event_loop only
updated last_event_id_ when the id was non-empty, so it could not tell
an empty id field from an event with no id field, and kept sending the
stale ID.
Track whether an id field was seen with a has_id flag, as has_data
does for the data field, and add a regression test.
eda9a10bfe
