title: Request origin not allowed: http://localhost:3001 when using Rails5 and ActionCable
body: <p>Having server issues with an app in Rails 5.0.0.beta2 trying to use ActionCable.</p>
  
  <p>Using localhost:3000 works fine, as that is what most of ActionCable defaults to. But if I try to run the rails server on port 3001, it gives me <code>Request origin not allowed: http://localhost:3001</code></p>
  
  <p>The ActionCable docs mention using something like <code>ActionCable.server.config.allowed_request_origins = ['http://localhost:3001']</code> which does work for me if I put it in <code>config.ru</code></p>
  
  <p>But that seems like a really weird place to put it. I feel like it should be able to go in an initializer file, or my development.rb environment config file.</p>
  
  <p>To further prove my point that it <em>should</em> be allowed to go in there, the setting <code>ActionCable.server.config.disable_request_forgery_protection = true</code> works to ignore request origin, even when I include it in development.rb.</p>
  
  <p>Why would <code>ActionCable.server.config.disable_request_forgery_protection</code> work in development.rb, but <code>ActionCable.server.config.allowed_request_origins</code> doesn't (but does work in config.ru)?</p>
  
  <p>Not a pressing issue, since I have several options as a work around. I just want to know if I'm missing something obvious about how I imagine this should be working.</p>