title: In Rails 4.2, How Does One Set the Response Content-Type Header With Media Type Parameters?
body: <p>In earlier versions, this would work:</p>
  
  <pre><code>ActionController::Renderers.add(:foo) do | data, options |
    self.content_type = 'application/foo; bar=1'
  end
  </code></pre>
  
  <p>In 4.2.4, this causes the Content-Type header to be blank. However, the following works, i.e., sets the Content-Type header to the string assigned to content_type:</p>
  
  <pre><code>ActionController::Renderers.add(:foo) do | data, options |
    self.content_type = 'application/foo'
  end
  </code></pre>
  
  <p>The other approach I know of, setting content_type on the render, seems to have no result anymore, i.e., <code>render('foo', content_type: 'application/foo')</code> doesn't set the header (never mind trying application/foo; bar=1.)</p>