Anchor the Action Cable server's route

Fixes https://github.com/rails/rails/issues/45489

- Adds `anchor: true` to the Action Cable server mount, so that it only strictly matches `/cable` rather than anything that starts with that.
- Uses `reverse_merge` instead of `merge` in `Mapper#mount`, so that you can override these options if you need to.

actioncable/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   The Action Cable server is now mounted with `anchor: true`.
+
+    This means that routes that also start with `/cable` will no longer clash with Action Cable.
+
+    *Alex Ghiculescu*
+
 *   `ActionCable.server.remote_connections.where(...).disconnect` now sends `disconnect` message
     before closing the connection with the reconnection strategy specified (defaults to `true`).
 

actioncable/lib/action_cable/engine.rb

@@ -51,7 +51,7 @@ module ActionCable
         config = app.config
         unless config.action_cable.mount_path.nil?
           app.routes.prepend do
-            mount ActionCable.server => config.action_cable.mount_path, internal: true
+            mount ActionCable.server => config.action_cable.mount_path, internal: true, anchor: true
           end
         end
       end

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -609,7 +609,7 @@ module ActionDispatch
           target_as       = name_for_action(options[:as], path)
           options[:via] ||= :all
 
-          match(path, options.merge(to: app, anchor: false, format: false))
+          match(path, { to: app, anchor: false, format: false }.merge(options))
 
           define_generate_prefix(app, target_as) if rails_app
           self

actionpack/test/dispatch/mapper_test.rb

@@ -172,6 +172,15 @@ module ActionDispatch
         assert_equal "/*path.:format", fakeset.asts.first.to_s
       end
 
+      def test_can_pass_anchor_to_mount
+        fakeset = FakeSet.new
+        mapper = Mapper.new fakeset
+        app = lambda { |env| [200, {}, [""]] }
+        mapper.mount app => "/path", anchor: true
+        assert_equal "/path", fakeset.asts.first.to_s
+        assert fakeset.routes.first.path.anchored
+      end
+
       def test_raising_error_when_path_is_not_passed
         fakeset = FakeSet.new
         mapper = Mapper.new fakeset