always enable HTML button (incorrectly announced Content-Type)

Please don't disable the HTML button for some mails.

The HTML button is disabled when viewing some mails. But some mails nevertheless contain HTML content which is displayed as such when using the classical Thunderbird View menu HTML option.

I guess this happens, if the mail incorrectly announces it's Content-Type: text/plain instead of Content-Type: text/html. Unfortunately a lot of mail systems seem to do that, especially when using Content-Transfer-Encoding: base64.

=> Please don't disable the button if no Content-Type: text/html is detected.