Raw HTML Export breaks umlauts

## Reproduction

When I add Umlauts like "Ü" in "ÜBUNG" to the markdown file,
And I click Export -> RAW HTML,
And I open the HTML file,
I can see "ÃœBUNG".
I would like to see "ÜBUNG".

## Error Analysis

I can see
```

```
in the exported file.

## Possible Solution

For me, it works with `utf-8`.
```

```

Related Issues:
- https://github.com/hackmdio/codimd/issues/135

Related Code:

https://github.com/hackmdio/codimd/blob/cb1e00d1d149c7a331e9d37cc73927a6aa5c7cf6/public/js/extra.js#L472
In the code, I do not know where the HTML `head` come from.