title: ActionCable unsubscribe callback not working when iOS client send "unsubscribe"
body: <p>Remote iOS client successfully connects to me, send subscribe command (it works fine), but on "unsubscribe" command I get next error:</p>
  
  <pre>
  Unsubscribing from channel: {"channel":"Assessor::StationChannel", "station_id": 1}
  Could not execute command from {"command"=>"unsubscribe", "identifier"=>"{\"channel\":\"Assessor::StationChannel\", \"station_id\": 1}"}) [NoMethodError - undefined method `unsubscribe_from_channel' for nil:NilClass]: /app/vendor/bundle/ruby/2.2.0/gems/actioncable-5.0.0/lib/action_cable/connection/subscriptions.rb:44:in `remove_subscription' | /app/vendor/bundle/ruby/2.2.0/gems/actioncable-5.0.0/lib/action_cable/connection/subscriptions.rb:40:in `remove' | /app/vendor/bundle/ruby/2.2.0/gems/actioncable-5.0.0/lib/action_cable/connection/subscriptions.rb:16:in `execute_command' | /app/vendor/bundle/ruby/2.2.0/gems/actioncable-5.0.0/lib/action_cable/connection/base.rb:88:in `dispatch_websocket_message' | /app/vendor/bundle/ruby/2.2.0/gems/actioncable-5.0.0/lib/action_cable/server/worker.rb:58:in `block in invoke' </pre>
  
  <p>Subscribe message format:</p>
  
  <pre>
  {"command": "subscribe", "identifier": "{\"channel\":\"Assessor::StationChannel\", \"station_id\": 1}"} </pre>
  
  <p>Unsubscribe message format:</p>
  
  <pre>
  {"command": "unsubscribe", "identifier": "{\"channel\":\"Assessor::StationChannel\", \"station_id\": 1}"} </pre>
  
  <p>I cannot reproduce this problem on localhost, so maybe somebody can help me?</p>