Fix instance_of? checks called on schema instance in dry-v

[flash-gordon]

@solnic ping me if you have any comments on this, this allows for passing injected deps into externally defined predicates

example code from dry-v

module IssuesPredicate
  include Dry::Logic::Predicates

  predicate(:equipment_exists?) do |account, value|
    account.include?(value)
  end
end

Schema = Dry::Validation.Schema do
  configure do
    option :account
    predicates(::IssuesPredicate)
  end

  optional(:equipment_ids).each { equipment_exists?(account) }
end

/cc @NikitaNaumenko