Share default mount path with client side .js

actioncable/app/assets/javascripts/action_cable.coffee.erb

@@ -5,7 +5,7 @@
   INTERNAL: <%= ActionCable::INTERNAL.to_json %>
 
   createConsumer: (url) ->
-    url ?= @getConfig("url") ? "/cable"
+    url ?= @getConfig("url") ? @INTERNAL.default_mount_path
     new ActionCable.Consumer @createWebSocketURL(url)
 
   getConfig: (name) ->

actioncable/lib/action_cable.rb

@@ -34,7 +34,8 @@ module ActionCable
       ping: 'ping'.freeze,
       confirmation: 'confirm_subscription'.freeze,
       rejection: 'reject_subscription'.freeze
-    }
+    },
+    default_mount_path: '/cable'.freeze
   }
 
   # Singleton instance of the server

actioncable/lib/action_cable/engine.rb

@@ -6,7 +6,7 @@
 module ActionCable
   class Railtie < Rails::Engine # :nodoc:
     config.action_cable = ActiveSupport::OrderedOptions.new
-    config.action_cable.mount_path = '/cable'
+    config.action_cable.mount_path = ActionCable::INTERNAL[:default_mount_path]
 
     config.eager_load_namespaces << ActionCable