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.

http://dev.rubyonrails.org/ticket/7457

Any feedback and/or support for the patch is appreciated

- Jason