gnome-software reads system proxy settings in a wrong way

After setting a proxy (e.g. http proxy) in Settings, gnome-software will show a message saying something like

`E: https://mirrors.ustc.edu.cn/ubuntu bionic/universe amd64 libkf5archive5 amd64 5.44.0-0ubuntu1 is not (yet) available (Unsupported proxy configured: 127.0.0.1://1234)`

when attempt to install software (not a snap package). That is to say, it parses the domain part as the protocol part, which is incorrect.

The proxy settings it shows seem to come from the system proxy settings as it shows

13:59:45:0380 GsPluginPackageKit Setting proxies (http: 127.0.0.1:1234, https: (null), ftp: (null), socks: (null), no_proxy: localhost,127.0.0.0/8,::1, pac: (null))

in `gnome-software --verbose`.