Generates and returns a numeric captcha image in base64 format. 
        Saves the correct answer in `session['captcha_answer']`
        Use later as:

        src = captcha.generate()
        <img src="{{src}}">

def generate(self):
        """
        Generates and returns a numeric captcha image in base64 format. 
        Saves the correct answer in `session['captcha_answer']`
        Use later as:

        src = captcha.generate()
        <img src="{{src}}">
        """                
        answer = self.rand.randrange(self.max)
        answer = str(answer).zfill(self.digits)        
        image_data = self.image_generator.generate(answer)
        base64_captcha = base64.b64encode(image_data.getvalue()).decode("ascii")
        logging.debug('Generated captcha with answer: ' + answer)
        session['captcha_answer'] = answer
        return base64_captcha