fix proxy setting loading

Summary:
when the flag isn't set we'd set the text to urlStr, urlStr however
was the original string *with* the port. since we want the port stripped
we need to actually set url.url() otherwise the KCM loading its own
settings would result in

Proxy: [socks://localhost:1080] Port: [1080]

which when written back to the file would result in broken data as we
do not sanitize the proxy url text WRT port when saving

Test Plan:

• set proxy
• close and open kcm again
• proxy values are what they were set to with no extra port duplication

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24558