title: ResearchKit optional and conditional step (survey task)
body: <p>How can I specify a step where user will be redirect to when he clicks on ‘skip‘ button in a step with predicate navigation rules ?</p>
  
  <p>For now it redirects to the start of the task.</p>
  
  <p>Here is the optional step :</p>
  
  <pre><code>// multiple choices question
  let multipleQuestionsStepTitle = "what is your gender ?"
  let textChoices = [ORKTextChoice(text: "Female", value: 0),
                     ORKTextChoice(text: "Male", value: 1),
                     ORKTextChoice(text: "idk", value: 2)]
  let multipleQuestionsAnswerFormat: ORKTextChoiceAnswerFormat = ORKAnswerFormat.choiceAnswerFormatWithStyle(.SingleChoice, textChoices: textChoices)
  let multipleQuestionsStep = ORKQuestionStep(identifier: "TextChoiceQuestionStep", title: multipleQuestionsStepTitle, answer: multipleQuestionsAnswerFormat)
  multipleQuestionsStep.optional = true
  
  steps += [multipleQuestionsStep]
  </code></pre>
  
  <p>And here is the navigation rule:</p>
  
  <pre><code>let task = ORKNavigableOrderedTask(identifier: "SurveyTask", steps: steps)
  
  var predicateRule: ORKPredicateStepNavigationRule
  let predicateFemale: NSPredicate = ORKResultPredicate.predicateForChoiceQuestionResultWithResultSelector(ORKResultSelector(resultIdentifier: "TextChoiceQuestionStep"), expectedAnswerValue: 0)
  // invert of previous predicate
  let predicateNotFemale: NSPredicate = NSCompoundPredicate.init(notPredicateWithSubpredicate: predicateFemale)
  predicateRule = ORKPredicateStepNavigationRule.init(resultPredicates: [predicateNotFemale], destinationStepIdentifiers: ["IntroStep"], defaultStepIdentifier: "ImageChoiceQuestionStep", validateArrays: false)
  
  task.setNavigationRule(predicateRule, forTriggerStepIdentifier: "TextChoiceQuestionStep")
  </code></pre>
  
  <p>I would assume that if we skip the step, we would be redirect to the <code>defaultStepIdentifier</code>, but it's not the case here.</p>
  
  <p>I didn't find anything in documentation about setting optional to false if we have navigation rules, so maybe someone has a solution on how to do that?</p>