xdg-settings set default-url-scheme-handler changes the entry for mime type text/html

From https://github.com/telegramdesktop/tdesktop/issues/16051

Reproduce:

1. Ensure that `text/html` in *$HOME/.config/mimeapps.list* is set to a known value, e.g. firefox.desktop.
2. Create a .desktop file named *telegram-desktop_telegram-desktop.desktop*. See below for an example file.
3. run `xdg-settings set default-url-scheme-handler tg telegram-desktop_telegram-desktop.desktop`
4. Now `xdg-open` or `gio open` will open any HTML file not in firefox, but with telegram.
5. in *$HOME/.config/mimeapps.list* the entry for `text/html` is changed to `text/html=telegram-desktop_telegram-desktop.desktop`.

Expected behaviour:
HTML files are opened in firefox or the app defined before.

Actual behaviour:
HTML files are opened in Telegram (which cannot handle this).

telegram-desktop is an example, and is how I experienced this issue. I'm not familiar enough with scheme-handlers and xdg-utils to be able to boil down a desktop-file to the bare minimals to reproduce this. Sorry for that.

*telegram-desktop_telegram-desktop.desktop* contents:
```
[Desktop Entry]
X-SnapInstanceName=telegram-desktop
Version=1.0
Name=Telegram Desktop
Comment=Official desktop version of Telegram messaging app
Exec=env BAMF_DESKTOP_FILE_HINT=/var/lib/snapd/desktop/applications/telegram-desktop_telegram-desktop.desktop /snap/bin/telegram-desktop -- %u
Icon=/snap/telegram-desktop/2551/meta/gui/icon.png
Terminal=false
StartupWMClass=TelegramDesktop
Type=Application
Categories=Chat;Network;InstantMessaging;Qt;
MimeType=x-scheme-handler/tg;
Keywords=tg;chat;im;messaging;messenger;sms;tdesktop;
```

I've gotten this file by installing telegram with snap and it is then located at */var/lib/snapd/desktop/applications/telegram-desktop_telegram-desktop.desktop*. I'm not sure if the desktop file works as it references to snap directories that do not exist if telegram-desktop is not installed.