The "updated" field should be able to be updated

[guanpu]

Is your feature request related to a problem? Please describe.
In my use case, I create feed at the beginning and add item gradually, this process can take very long so that at the end, the updated is no longer accurate. I think the updated of the feed should be equal to the last added item's updated field. But at the beginning, I don't know what's last item's update time. So there's no way to make updated field of feed accurate.

Describe the solution you'd like
Expose a setUpdated method so that user can update it to be anytime they think valid

Describe alternatives you've considered
Here's my implementation: #143

Additional context
Maybe my usecase is special, but I guess generally we should expose this method.