title: Unique IVs producing identical ciphertext using attr_encrypted
body: <p>Using the most basic setup:</p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    attr_encrypted :name, 
                   key: 'This is a key that is 256 bits!!', 
                   encode: true, 
                   encode_iv: true, 
                   encode_salt: true
  end
  </code></pre>
  
  <p>The results look like this in the database when supplying an identical name:</p>
  
  <pre><code>╔════╦══════════════════════════════╦═══════════════════╗
  ║ id ║ encrypted_name               ║ encrypted_name_iv ║
  ╠════╬══════════════════════════════╬═══════════════════╣
  ║ 1  ║ aVXZb1b317nroumXVBdV9pGxA2o= ║ JyE7wHups+3upY5e  ║
  ║ 2  ║ aVXZb1b317nroumXVBdV9pGxA2o= ║ uz/ktrtbUAksg5Vp  ║
  ╚════╩══════════════════════════════╩═══════════════════╝
  </code></pre>
  
  <p>Why is the ciphertext identical? Isn't that the part of the point of iv, which the gem is using by default?</p>