handle existing certificate endpoint at heroku

[OleMchls]

I tried to set this up for slidr.io. But encoutered a HTTP 500. The log say:

2017-02-08T21:23:06.925074+00:00 app[web.1]: 21:23:06.924 [error] #PID<0.617.0> running HerokuConnector.Endpoint terminated
2017-02-08T21:23:06.925078+00:00 app[web.1]: Server: addon-heroku-connector.herokuapp.com:80 (http)
2017-02-08T21:23:06.925079+00:00 app[web.1]: Request: POST /connections/54/connect
2017-02-08T21:23:06.925079+00:00 app[web.1]: ** (exit) an exception was raised:
2017-02-08T21:23:06.925082+00:00 app[web.1]:     ** (KeyError) key :cname not found in: %Happi.Heroku.Error{code: 422, id: "invalid_params", message: "Only one SNI endpoint is allowed per app (try certs:update instead).", url: ""}
2017-02-08T21:23:06.925084+00:00 app[web.1]:         (heroku_connector) web/services/connection_service.ex:48: HerokuConnector.ConnectionService.connect_with_certificate/4
2017-02-08T21:23:06.925084+00:00 app[web.1]:         (heroku_connector) web/controllers/connection_controller.ex:48: HerokuConnector.ConnectionController.connect/2
2017-02-08T21:23:06.925085+00:00 app[web.1]:         (heroku_connector) web/controllers/connection_controller.ex:1: HerokuConnector.ConnectionController.action/2
2017-02-08T21:23:06.925085+00:00 app[web.1]:         (heroku_connector) web/controllers/connection_controller.ex:1: HerokuConnector.ConnectionController.phoenix_controller_pipeline/2
2017-02-08T21:23:06.925086+00:00 app[web.1]:         (heroku_connector) lib/heroku_connector/endpoint.ex:1: HerokuConnector.Endpoint.instrument/4
2017-02-08T21:23:06.925087+00:00 app[web.1]:         (heroku_connector) lib/phoenix/router.ex:261: HerokuConnector.Router.dispatch/2
2017-02-08T21:23:06.925087+00:00 app[web.1]:         (heroku_connector) web/router.ex:1: HerokuConnector.Router.do_call/2
2017-02-08T21:23:06.925088+00:00 app[web.1]:         (heroku_connector) lib/heroku_connector/endpoint.ex:1: HerokuConnector.Endpoint.phoenix_pipeline/1