Previously, the event's `time` property could be either a string or a date.
this commit modifies that to ensure that the object can only be created with
a timestamp in string format. As long as the string is a valid date, that
can be parsed by `new Date(Date.parse(str))` then whenever the event is
serialized as JSON, the `time` attribute will be formatted as per RFC 3339.
Fixes: https://github.com/cloudevents/sdk-javascript/issues/326
Signed-off-by: Lance Ball <lball@redhat.com>
Remi Cattiau
Lance Ball
Grant Timmerman