QDesktopServices::openUrl does not properly encode mailto: urls before passing them to the protocol handler on windows

QDesktopServices::openUrl passes the string representation of the url to the program registered for the mailto: url instead of the properly encoded variant. This leads to problems if the url contains special characters (In our case we tried to prepopulate the message body with a few lines, thunderbird completely ignored the linebreaks, "The Bat" only showed the first line). With the attached patch everything worked as expected.