Wrong text encoding in preview text

This issue has been migrated from [#2891](https://github.com/matrix-org/synapse/issues/2891).

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### Description

Wrong text encoding is used in the preview text on some sites.

### Steps to reproduce

- post an url like http://www.interfax.ru/russia/600675
- the preview text looks like `"Ïåðâûé êàíàë" îïðîâåðã èíôîðìàöèþ îá óõîäå Åêàòåðèíû Àíäðååâîé èç ïðîãðàììû "Âðåìÿ" - Interfax.ru
Àíäðååâà âåäåò ïðîãðàììó áîëåå 20 ëåò è âñå åùå îñòàåòñÿ åå âåäóùåé` which is obviously wrong, it's CP-1251 text decoded as CP-1252. The page contains `` so browsers display it fine.
- try another page from the same site, for example http://www.interfax.ru/russia/
- the text is decoded correctly and reads `В России - Новости Интерфакс
Новости в России.`

The difference seems to be in the response headers. Where the text is decoded correctly the Content-Type returned by the server is `text/html; Charset=windows-1251`. Where it's wrong it's just `text/html`. I think the `http-equiv` value should have precedence over the header (if it's parsed at all).

### Version information

- **Homeserver**: own homeserver
- **Version**: Synapse/0.26.0
- **Install method**: package manager
- **Platform**: set up on Debian 9.3 amd64.