Add charset to content-type in headers.

This is not a standard thing to do in POST request. See comment in code for detail explenation why we are doing it.
* Radsej `Content-Type` nez `content-type`.
* Chcel som povedat ze dlhy riadok ale 87 znakov nie je az tak vela a rozdelene je to skaredsie, takze nevermind ;-)
* Navrhujem kratsi komentar:
```
        # WebUIServlet needs charset set in Content-Type. Normal POST requests
        # don't have it, but request.html manually sets it, so we do the same.
```