title: Rails 4 Validation: where to put the allow_nil when doing an inclusion?
body: <p>Are these two implementations functionally equivalent? If so, which is "better?"</p>
  
  <pre><code>  # from a model
    WIDGET_COLORS = %w(red yellow green)
    validates :widget_color,
             inclusion: {in: WIDGET_COLORS, allow_nil: true}
  </code></pre>
  
  <p>or</p>
  
  <pre><code>  # from a model
    WIDGET_COLORS = %w(red yellow green)
    validates :widget_color,
             inclusion: {in: WIDGET_COLORS},
             allow_nil: true
  </code></pre>
  
  <p>UPDATE: fixed typo so example reads <em>validates</em></p>