title: Mongomapper :minimum & :maximum vs. :length
body: <p>I try to validate in MongoMapper a string with:</p>
  
  <pre><code>key :title,       String, :require =&gt; true, :length =&gt; 4..30
  </code></pre>
  
  <p>And I got always the error "title is too short (minimum is 4 characters)" also when the string was longer than 4 chars.</p>
  
  <p>If I try it with  </p>
  
  <pre><code>key :title,       String, :require =&gt; true, :minimum =&gt; 4, :maximum =&gt; 30
  </code></pre>
  
  <p>and it work as excepted. Can someone explain why this happen or why the first thing is wrong?</p>