title: ActiveRecord::MultiparameterAssignmentErrors when a parameter was left blank in date_select
body: <p>I'm having trouble validating date fields using date_select helper from form_builder, whenever one of the parameters is left blank (e.g year params). I'm using the validates_timeliness gem and it works perfectly with datetime_select however it doesn't work on date_select.</p>
  
  <p>on my model class I do have this validation:</p>
  
  <pre><code>validates_date :start_date, on_or_before: :today, allow_blank: true
  </code></pre>
  
  <p>this is the field on the view (uses simple_form)</p>
  
  <pre><code>= f.input :start_date
  </code></pre>
  
  <p>and this are the parameters (start_date(1i) is for year params and was left blank):</p>
  
  <pre><code>"start_date(1i)"=&gt;"",
  "start_date(2i)"=&gt;"2",
  "start_date(3i)"=&gt;"3"
  </code></pre>
  
  <p>and this is what I got after submitting it</p>
  
  <pre><code>1 error(s) on assignment of multiparameter attributes [error on assignment [nil, 2, 3] to start_date (undefined method `&lt;' for nil:NilClass)]
  </code></pre>
  
  <p>Hope somebody can help me out with this. Thank you in advance.</p>