title: rails validation: :allow_nil and :inclusion both needed at the same time
body: <p>Usually the field 'kind' should be allowed blank.
  but if it is not blank, the value should included in ['a', 'b']</p>
  
  <pre class="lang-rb prettyprint-override"><code>validates_inclusion_of :kind, :in =&gt; ['a', 'b'], :allow_nil =&gt; true
  </code></pre>
  
  <p>The code does not work?</p>