Add missing validation rule to poll answers

We were checking we didn't have more votes than allowed in the case of
questions with multiple answers, but we weren't checking it in the case
of questions with a single answer. This made it possible to create more
than one answer to the same question. This could happen because the
method `find_or_initialize_user_answer` might initialize two answers in
different threads, due to a race condition.

app/models/poll/question.rb

@@ -28,7 +28,7 @@ class Poll::Question < ApplicationRecord
   accepts_nested_attributes_for :question_options, reject_if: :all_blank, allow_destroy: true
   accepts_nested_attributes_for :votation_type
 
-  delegate :max_votes, :multiple?, :vote_type, to: :votation_type, allow_nil: true
+  delegate :multiple?, :vote_type, to: :votation_type, allow_nil: true
 
   scope :by_poll_id,    ->(poll_id) { where(poll_id: poll_id) }
 
@@ -90,6 +90,14 @@ class Poll::Question < ApplicationRecord
     votation_type.nil? || votation_type.unique?
   end
 
+  def max_votes
+    if multiple?
+      votation_type.max_votes
+    else
+      1
+    end
+  end
+
   def find_or_initialize_user_answer(user, title)
     answer = answers.find_or_initialize_by(find_by_attributes(user, title))
     answer.answer = title