title: Json Schema validation: do not allow fields other than those declared in schema
body: <p>Suppose that I have schema like</p>
  
  <pre><code>fname: string
  lname: string
  age: string
  </code></pre>
  
  <p>None of them are required. User can send me any of those attributes above but nothing else that is not declared. They can pass me <code>fname</code>, <code>lname</code> and <code>age</code> or all. But if they pass me all and additional property like <code>middle_name</code> the message should be rejected.</p>
  
  <p>How would I define a schema like this?</p>