title: url_for not using default_url_options[:host] value
body: <p>I've got a view for an ActionMailer that includes a few different links. I'm running it on localhost:3000 right now, and so I've set that as such in a file called <code>setup_mail.rb</code> in app/initializers (as indicated <a href="http://railscasts.com/episodes/206-action-mailer-in-rails-3?view=asciicast" rel="noreferrer">here</a>):</p>
  
  <pre><code>ActionMailer::Base.default_url_options[:host] = "localhost:3000"
  </code></pre>
  
  <p>When I go to use <code>url_for</code> in the view, it doesn't seem to pull this value. If I then add <code>:host =&gt; "localhost:3000"</code> to each <code>url_for</code> tag, they work properly. But they don't work without that included.</p>
  
  <p>I have another tag, <code>project_url</code>, which is as it appears: a link to a specified Project. This functions, including the host value, with just <code>project_url(@project)</code>. Why would one work but not the other?</p>
  
  <p>From everything I've read, setting the <code>default_url_options[:host]</code> in an initializer should allow me to omit the <code>:host</code> value in the <code>url_for</code> tag. Obviously, it's not the worst thing in the world to just add that value, but it seems unnecessary and it means that when I eventually host the project somewhere I'll have to go through and change that value all over the place. But worse than that, it's something that I don't understand. I'm still learning as I go here and so I'd like to know what I'm doing wrong.</p>