custom proxy factories not effective when system proxy is set

[JanKirchhoff]

Creating a custom proxy factory is not effective when a system proxy is set.
This is an issue when you need to use a special proxy e.g. to get access to a web ressource that is not available via the system proxy.

It appears that this happens due to the fact that

if ( !systemproxies.isEmpty() )

is executed before

if ( !proxies.isEmpty() )

which would return an inserted proxy.

QGIS/src/core/network/qgsnetworkaccessmanager.cpp

Lines 79 to 88 in f8dbfdd

	for ( QNetworkProxyFactory *f : constProxyFactories )
	{
	QList<QNetworkProxy> systemproxies = QNetworkProxyFactory::systemProxyForQuery( query );
	if ( !systemproxies.isEmpty() )
	return systemproxies;
	QList<QNetworkProxy> proxies = f->queryProxy( query );
	if ( !proxies.isEmpty() )
	return proxies;
	}