Fix favicon download from URL with non-standard port.

Fixes #5001.

The favicon download URL was constructed from scheme and host only. This is fixed by simply replacing the path of the original URL with "/favicon.ico", thus keeping scheme, host, auth and port intact.

Further modification: URL's with a non-http schema are now rejected.

tests/TestIconDownloader.h

@@ -0,0 +1,35 @@
+/*
+ *  Copyright (C) 2011 Felix Geyer <debfx@fobos.de>
+ *  Copyright (C) 2019 KeePassXC Team <team@keepassxc.org>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 2 or (at your option)
+ *  version 3 of the License.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef KEEPASSXC_TESTICONDOWNLOADER_HPP
+#define KEEPASSXC_TESTICONDOWNLOADER_HPP
+
+#include <QList>
+#include <QObject>
+#include <QString>
+
+class TestIconDownloader : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void testIconDownloader();
+    void testIconDownloader_data();
+};
+
+#endif // KEEPASSXC_TESTICONDOWNLOADER_HPP