title: Rails: validates_length_of :allow_nil does not allow nil
body: <p>Using: Rails 3.0.3.</p>
  
  <p>I am using this code in the validator:</p>
  
  <pre><code>validates_length_of :birth_date_8, :minimum =&gt; 8, :allow_nil =&gt; true, :message =&gt; "value_is_not_a_proper_date", :if =&gt;:family_birthday? 
  </code></pre>
  
  <p>It does not, however, allow the field to be nil. I have checked so it is not a matter of the <code>:if =&gt; :family_birthday?</code> that is messing things up.</p>
  
  <p>Is there something wrong with the code?</p>