'Provides hints to the placeholder, and in turn to the page for
determining VARY headers for the response.
Must return one of:
- None (default),
- String of a case-sensitive header name, or
- iterable of case-sensitive header names.
NOTE: This only makes sense to use with caching. If this plugin has
``cache = False`` or plugin.get_cache_expiration(...) returns 0,
get_vary_cache_on() will have no effect.'

def get_vary_cache_on(self, request, instance, placeholder):

    return None
