title: Can't use facebook to authenticate: undefined method 'web_server' for OAuth2?
body: <p>Trying to let users sign in / sign up with Twitter and Facebook. Twitter works no problem but the strategy is different for Facebook. </p>
  
  <pre><code>undefined method `web_server' for #&lt;OAuth2::Client:0x00000005211d58&gt;
  </code></pre>
  
  <p>Trace shows</p>
  
  <pre><code>oa-oauth (0.0.1) lib/omniauth/strategies/oauth2.rb:18:in `request_phase'
  oa-oauth (0.0.1) lib/omniauth/strategies/facebook.rb:28:in `request_phase'
  oa-core (0.0.5) lib/omniauth/strategy.rb:25:in `call!'
  oa-core (0.0.5) lib/omniauth/strategy.rb:19:in `call'
  oa-core (0.0.5) lib/omniauth/builder.rb:22:in `call'
  warden (1.0.5) lib/warden/manager.rb:35:in `block in call'
  warden (1.0.5) lib/warden/manager.rb:34:in `catch'
  warden (1.0.5) lib/warden/manager.rb:34:in `call'
  </code></pre>
  
  <p>Anybody else experienced this?</p>
  
  <p>ps. I'm using the following gems: </p>
  
  <pre><code>gem 'oa-oauth', :require =&gt; 'omniauth/oauth'
  gem 'oauth2'
  </code></pre>
  
  <p>I'm not using the full omniauth gem as its addressable dependencies conflict with other gems.</p>