title: ng-minlength is valid when length is 0
body: <p>I'm new to angular and stumbled upon a validation problem in a form.
  Below is my input field and when I don't enter any value it show's as valid and also 'ng-valid' class is added to it even though I've defined minlength to be 5</p>
  
  <pre><code>&lt;input id="name" type="text" placeholder="Name" ng-model="formdata.name" ng-minlength=5 /&gt;
  </code></pre>
  
  <p>It happens even in angular example - <a href="http://docs.angularjs.org/api/ng.directive:input" rel="noreferrer">http://docs.angularjs.org/api/ng.directive:input</a></p>
  
  <p>How can I resolve this?</p>