Learn that the statement provided is a valid response.

def learn_response(self, statement, previous_statement=None):
        """
        Learn that the statement provided is a valid response.
        """
        if not previous_statement:
            previous_statement = statement.in_response_to

        if not previous_statement:
            previous_statement = self.get_latest_response(statement.conversation)
            if previous_statement:
                previous_statement = previous_statement.text

        previous_statement_text = previous_statement

        if not isinstance(previous_statement, (str, type(None), )):
            statement.in_response_to = previous_statement.text
        elif isinstance(previous_statement, str):
            statement.in_response_to = previous_statement

        self.logger.info('Adding "{}" as a response to "{}"'.format(
            statement.text,
            previous_statement_text
        ))

        # Save the input statement
        return self.storage.create(**statement.serialize())