[PERSISTANCE] fixed getter methods for underscore-options of unsaved rec...

...ords

For example It fixes this live exception:
[37] pry(main)> PersistentArticle.new(:id => 'someId').persisted?
NoMethodError: undefined method `[]' for nil:NilClass
from /Users/AlexY/.rvm/gems/ruby-2.0.0-p353@startwire/gems/tire-0.6.2/lib/tire/model/persistence.rb:48:in `block (3 levels) in included'