[wkWebView] wkWebView disables JS execution when app is backgrounded

When using wkWebView, Javascript execution is totally stopped after the app is backgrounded by the user. This is a major problem for apps using background modes as no javascript callback will be executed unless the app is either in the foreground or was already terminated by the user / OS.

example: cordova-plugin-geofence's javascript callbacks will not trigger after the app was backgrounded but will however trigger if the app is in the foregrounded or was killed by the user / OS.

UIView does not have this problem.