'Returns a plain JSON-encoded string with content-type
``application/json``. The content-type may be overridden by
setting ``request.response.content_type``.'

def __call__(self, info):

    def _render(value, system):
        request = system.get('request')
        if (request is not None):
            response = request.response
            ct = response.content_type
            if (ct == response.default_content_type):
                response.content_type = 'application/json'
        default = self._make_default(request)
        return self.serializer(value, default=default, **self.kw)
    return _render
