better proxy support

[Xavier Vello]

Hello

In KDE, it's impossible to use a proxy only for http (and not https nor ftp), 
which is quite annoying, especially for ftp.
The attached patch disables proxy for http/https when the proxy url is empty 
(no host), allowing to use proxy only for http and direct https. Please review 
it before I commit it.

I can't test ftp proxy support, is someone willing to do the same patch for 
ftp ? (checking proxyUrl.hasHost() should be enough).

Regards
-- 
Xavier Vello
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