fix revision access failing when limit is enabled

[dbil25]

When limit is enabled and reached, if a field was initialized but never changed, it will be merged and result in a audit like that:

action: "update", audited_changes: {"name" => "Brandon", "username" => ["brandon", "keepers"]}

(notice the action: "update" and the value in "name" beeing a single value, not an array)

and then, when trying to access a revision, we get this error:

undefined method `last' for "Brandon":String
        attrs[attr] = (action == "update" ? values.last : values)

So in this fix, i added a failing test, then fixed it by changing
attrs[attr] = (action == "update" ? values.last : values) to
attrs[attr] = (values.is_a?(Array) ? values.last : values)