title: How to get the auth URL from an OmniAuth provider without redirect
body: <p>How do you get the URL that OmniAuth redirect to when you go to <code>/auth/twitter</code>?</p>
  
  <p>I am building an API server and just want to pass the URL in JSON to the client so it can do whatever it needs to with the URL.</p>
  
  <p>I think what I want is the result of the strategy's <code>request_phase</code> as that looks to be the auth URL, but how do I get that in a new controller so I can do:</p>
  
  <pre><code>class MyOauthController &lt; ApplicationController
    # GET /my_oauth/signin/twitter.json
    def signin
      url = ???
      render json: {signin_url: url}
    end
  </code></pre>
  
  <p>Is there a clean way to do this?</p>