is_valid重写。返回数据格式统一

    def is_valid(self, raise_exception=False):
        assert hasattr(self, 'initial_data'), (
            'Cannot call `.is_valid()` as no `data=` keyword argument was '
            'passed when instantiating the serializer instance.'
        )
        if not hasattr(self, '_validated_data'):
            try:
                self._validated_data = self.run_validation(self.initial_data)
            except validators.ValidationError as exc:
                self._validated_data = {}
                self._errors = exc.detail
            else:
                self._errors = {}
        if self._errors and raise_exception:
            errors = ['_'.join([key, value[0]]) for key, value in self._errors.items()][0]
            raise ParseError(errors)
        return not bool(self._errors)