title: devise/omniauth login with twitter - validation failed: email can't be blank
body: <p>I've set up authentication in a rails app using devise, following <a href="http://railscasts.com/episodes/209-introducing-devise?view=asciicast" rel="noreferrer">this tutorial</a>.
  It works great, a user can register and then log in using an email and password. </p>
  
  <p>I now want to add the ability to register/log in using twitter. I'm following the guide from the <a href="https://github.com/plataformatec/devise/wiki/OmniAuth%3a-Overview" rel="noreferrer">devise wiki</a></p>
  
  <p>The example in the wiki uses facebook, I've changed it to use twitter. It manages to go to twitter and request authorisation, so I'm sure I have the basics working. However once it returns to my app I get the following error.</p>
  
  <pre><code>Validation failed: Email can't be blank
  </code></pre>
  
  <p>I presume it's because I'm failing to provide an email address. </p>
  
  <p>The code I'm using is identical (apart from swapping facebook to twitter) to that in the tutorials, so I'm not posting any samples. Please let me know if you want to see anything. </p>
  
  <p>I'm quite new to rails so I'm not sure how to debug. Any advice on how I should continue?</p>