title: password with minimum 6 character doesn't work [rails 4]
body: <p>I am reading the book of Agile web development with rails 4</p>
  
  <p>in the user model I have a password validation that cannot be less than 6 character</p>
  
  <pre><code>validates :password_digest, length: {minimum: 6}
  </code></pre>
  
  <p>But actually when I create a new user, it allow me to do it with less than 6. Any suggestions?</p>
  
  <p>Thanks</p>