Prevent interpreting URL parameter for Qt plugins as content URLs.

This prevents a compatibility workaround for NPAPI plugins from
interfering with the Qt plugins.

Task-number: QTBUG-39483
Change-Id: Ic9342d3bdeec4c26c21f56b475d3d3751b30f057
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>