title: rspec should ensure_length_of allow_nil
body: <p>Model</p>
  
  <pre><code>validates_length_of :description, :maximum =&gt; 255, :allow_nil =&gt; true
  </code></pre>
  
  <p>spec_file</p>
  
  <pre><code>it { should ensure_length_of(:description).is_at_most(255).allow_nil }
  </code></pre>
  
  <p>return exeption</p>
  
  <pre><code>Failure/Error: it { should ensure_length_of(:description).is_at_most(255).allow_nil }
   NoMethodError:
     undefined method `allow_nil' for #&lt;Shoulda::Matchers::ActiveModel::EnsureLengthOfMatcher:0x0000000acb03e0&gt;
  </code></pre>
  
  <p>Pls, help!</p>