seal() options is not guarded against mutation

[kanongil]

If you want to change the ttl on each invocation, you can't re-use the same object since the value is not extracted from the options object until after an await.

Example:

const config = Object.assign({}, Iron.defaults);

config.ttl = 42;
const promise = Iron.seal(object, secret, config);
config.ttl = 4200;
const sealed = await promise;

The resulting object will be sealed with a ttl == 4200.

I suspect there are similar issues with unseal() as well.