Fixing onContentPrepareForm for multilang sites

When LanguageFilter plugin is enabled, the data object containing the form data (specified at http://docs.joomla.org/Plugin/Events/Content#onContentPrepareForm) gets converted to an array containing the data for some components, which results in inability to manipulate it as it isn't passed by reference.

This PR removes conversion to array and restores passing the data as an object, which seems to be the correct way.

To test, just make sure that multilingual assotiations/content still works fine when you are editing Content/Categories/Contacts/Newsfeeds