FIX Changing `alterCanIncludeInGoogleSitemap` to call owner object and extensions

At the moment `alterCanIncludeInGoogleSitemap` is only called on other extensions, but really we'd want to see this called on the owner object and its extensions.

Fixed to use `invokeWithExtensions` rather than `extend`
Merge pull request #52 from TomSpeak/master

FIX Changing `alterCanIncludeInGoogleSitemap` to call owner and extensions