Generate standard request headers

def request_headers(self, request):
        """
        Generate standard request headers
        """
        headers = {}

        allow_origin = self.allow_origin(request)
        if allow_origin:
            headers = dict_filter({
                'Access-Control-Allow-Origin': allow_origin,
                'Access-Control-Allow-Credentials': {True: 'true', False: 'false'}.get(self.allow_credentials),
                'Access-Control-Expose-Headers': ', '.join(self.expose_headers) if self.expose_headers else None,
            })

        return headers