fix: issue with delete user

[savon-noir]

the user delete api call fails on vaultwarden when "content-type: app/json" is not provided in http headers:

>>> client.delete(u.Id)
ERROR:vaultwarden.utils.logger:Error: 404
WARNING:vaultwarden.utils.logger:Failed to <redacted user.id> Client error '404 Not Found' for url 'https://<redacted>/admin/users/<redacted user.id>/delete'
For more information check: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/404
False

This PR fixes that issue. It is an easy fix. You can also change it yourself on the next release and discard this PR.