Can not detect https with CloudFlare Flexible SSL

[Ovear]

Hello, i found that it seems that NextCloud can not work with CloudFlare Flexible SSL fine after a unknown update. When i use Flexible SSL, NextCloud still use http://domain as a base domain, which caused "Content Security Policy" failed.
This solution is to use "X-Forwarded-Proto" to detect protocol, but NextCloud won't trust this at default, and 'trusted_proxies' section doesn't support CIRD which means there is no good method to add any public cdn ip range to this list.
#6550

Steps to reproduce

1. Turn on CloudFlare Flexible
2. Open your website
3. That's it

Expected behaviour

NextCloud use https://domain as a base domain.

Actual behaviour

NextCloud use http://domain as a base domain.

Server configuration

Operating system: Centos 6

Web server: Apache 2.4

Database: MySQL 5.7

PHP version: 5.6.30

Nextcloud version: (see Nextcloud admin page) 12.0.3

Updated from an older Nextcloud/ownCloud or fresh install: No

Where did you install Nextcloud from: Official website

Signing status:

Signing status

[MorrisJobke]

Please use the overwrite config options:

https://github.com/nextcloud/server/blob/master/config/config.sample.php#L391-L406

and

https://github.com/nextcloud/server/blob/master/config/config.sample.php#L427-L435

If you need further help with your setup, please ask in the forums for help first: https://help.nextcloud.com/