title: Joi Validation - Field is not required when the condition is not met
body: <p>I am writing an update endpoint in nodeJs. I want to make sure that additional fields wont be allowed, if certain flags are checked.</p>
  <pre><code>all_restaurant: Joi.boolean().required(),
  merchant_id: Joi.when('all_restaurant',{'is':false, 
                  then: Joi.string().required(), otherwise:Joi.disallow()})
  </code></pre>
  <p>If all_restaurant is true, I need to throw an error when the user tries to include any merchant_id, but it still allowing me to update.
  I tried using strict() after the field and in the top level but nothing is working. is there any way to achieve that?</p>