title: What's the variable HTTP_X_FORWARDED_HOST in the @env hash in middleware?
body: <p>In a Rack middleware filter, I know the call method takes an env hash variable.  I'm looking through someone's Rack middleware code and I see</p>
  
  <pre><code> env['HTTP_X_FORWARDED_HOST']
  </code></pre>
  
  <p>Can someone explain what HTTP_X_FORWARDED_HOST means?  Is it the host where the request originated from?</p>