Fix undefined method error when voting up

<pre>
NoMethodError (undefined method `id' for true:TrueClass):
  plugins/redmine_vote/lib/acts_as_voteable.rb:60:in `block in voted_by_user?'
  vendor/bundle/ruby/1.9.1/gems/activerecord-3.2.19/lib/active_record/associations/collection_proxy.rb:91:in `each'
  vendor/bundle/ruby/1.9.1/gems/activerecord-3.2.19/lib/active_record/associations/collection_proxy.rb:91:in `method_missing'
  plugins/redmine_vote/lib/acts_as_voteable.rb:59:in `voted_by_user?'
  plugins/redmine_vote/lib/acts_as_voteable.rb:32:in `vote'
</pre>