Update a book

            The "body" is merged with the current one.
            Yes, it is NOT overwritten.

            In case of concurrency conflict
            this function could raise `elasticsearch.ConflictError`

def update_book(self, id, body, doc_type='book'):
        ''' Update a book

            The "body" is merged with the current one.
            Yes, it is NOT overwritten.

            In case of concurrency conflict
            this function could raise `elasticsearch.ConflictError`
        '''
        # note that we are NOT overwriting all the _source, just merging
        book = self.get_book_by_id(id)
        book['_source'].update(body)
        validated = validate_book(book['_source'])
        ret = self.es.index(index=self.index_name, id=id,
                            doc_type=doc_type, body=validated, version=book['_version'])
        return ret