Issue with "=" at the end of a koa-session encoded property 
Hi! 

I'm using node version 20.10.0.

``` 
$ nvm list
       v18.17.1
       v18.18.1
->     v20.10.0
``` 
I've assigned an object to a session property:

` ctx.session.user = loginData.user; `

The koa-session encoding default function terminate the string with a `=`. This is not liked here:
``` js 
//  node_modules/cookies/index.js

var RESTRICTED_CHARS_REGEXP = /[;=]/

[...]

if (value && (!fieldContentRegExp.test(value) || RESTRICTED_CHARS_REGEXP.test(value))) {
    throw new TypeError('argument value is invalid');
}
``` 
as I get the following error:
``` 
[2024-01-01T23:08:12.586] [ERROR] stdout - TypeError: argument value is invalid
    at new Cookie (/Users/maurizio/projects/ewine/backend/node_modules/cookies/index.js:152:11)
    at Cookies.set (/Users/maurizio/projects/ewine/backend/node_modules/cookies/index.js:111:16)
    at ContextSession.save (/Users/maurizio/projects/ewine/backend/node_modules/koa-session/lib/context.js:341:22)
    at ContextSession.commit (/Users/maurizio/projects/ewine/backend/node_modules/koa-session/lib/context.js:244:16)
    at session (/Users/maurizio/projects/ewine/backend/node_modules/koa-session/index.js:46:20)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
``` 
Any help on how to solve this? Thank you!

