Enhancement request: would be nice to have Page#validate method
use an additional argument - a mask with only the fields to
validate against.
Say, I am creating a user and I have a hash with all user
parameters, and on a user creation page I need to validate only a
couple of fields, then I could pass to the validate method the
whole user hash as first argument, and a mask with the fields to
validate against, without me having first to remove the fields I
dont care validating against from the user hash.
