Allowing validates_acceptance_of to store value on the database column

I've reopened a ticket for this and provided a patch withs tests. From my notes on the ticket:

> An example use case here, is that I want to use the current version of the EULA as the :accept value, and store that column, so I can refer to the explicit version the user accepted at a later date.

Any feedback and/or support for the patch is appreciated

- Jason