`Query#update` doesn't allow *not* updating the `updated_at` field

[asuntskel]

Right now, batch Query#update, always changes updated_at.

I totally understand the reason why it was added (#1087 and #1091), but I also think it would be useful to have an option to opt-in / out.

Since at first I thought doing column: IGNORE would do something like this (wrong), I was thinking of a solution such as

Query.update(column: value, updated_at: UNCHANGED) # so you can specifically opt-out of auto updating
# or something like...
Query.update_only(column: value) # only available on tables that have have the timestamps

That would be useful for system-wide tasks / admin stuff and batch updating models with a lot of columns.
I wonder what anyone else thinks of this? Let me know :)