Lose data when go offline while typing

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

Sometimes, while I'm typing, the status (in the top right of the page) changes to `offline`.

When it comes back to `online`, if I continue typing, sometimes it loses some text that I typed when it was `offline`.

#### Current behaviour

Steps to reproduce:

1. Go to text editing
2. Type some text
3. Disconnect your pc from internet
4. Continue typing until Hackmd blocks the typing
5. Reconnect to the internet
6. Then you will lose some text you typed

#### Expected behaviour

To not lose text when it comes back to online

#### Environment

I use HackMD on:

**Desktop**
- OS: ubuntu 16.04 LTS
- Browser: chrome
- Browser Version: 71