Getter/Setter does not work with ES6 class in extend/context.ts
## What happens?
Context Getter/Setter does not work with ES6 class.


        class Context {
            constructor() {
            }
            /**
             ** - Return all operators of Sequelize
             ** @url https://demopark.github.io/sequelize-docs-Zh-CN/querying.html
             **/
            get Op() {
                return this.app.Sequelize.Op;
            }
            get print() {
                return this.body;
            }
            get realIP() {
                return this.get['X-Real-IP'] || this.get['X-Forwarded-For'] || this.ip;
            }
            /**
             ** @param {*} value
             ** @example
             ** ctx.print = 'Hello'
             ** ctx.print = null
             ** ctx.print = {name: 'Hello'}
             ** @returns {void}
             **/
            set print(value) {
        
                const body = {
                    data: value,
                    errorCode: 0,
                    msg: ERROR_CODE[0],
                    success: true
                };
        
                if (isPlainObject(value)) {
        
                    if (value.errorCode !== undefined && value.errorCode !== 0) {
                        body.errorCode = value.errorCode;
                        body.success = false;
                        body.msg = ERROR_CODE[body.errorCode] || 'error';
                        body.data = null;
                    }
        
                    body.msg = value.msg || body.msg;
                    body.errorMsg = value.errorMsg || body.msg;
        
                    // Delete duplicate defined fields
                    if (isPlainObject(body.data)) {
                        body.data = Object.assign({}, body.data);
                        delete body.data.errorCode;
                        delete body.data.msg;
                        delete body.data.success;
                    }
                }
        
                this.body = body;
            }
        }
        
        // export default new Context();
        
        module.exports = new Context();


**Steps to reproduce the behavior:**
1. The setter and getter do not run when I use ctx.print = <...something...>

**Expected behavior**
1. It should Run

## Context
- **Node Version**: 14
- **Egg Version**: 2.30
- **Platform**: NodeJS
