title: json schema date-time does not check correctly
body: <p>I have a JSON and a JSON-schema</p>
  
  <p>JSON:</p>
  
  <pre><code>{
  "aaa": "4000-02-01 00:00:00"
  }
  </code></pre>
  
  <p>JSON-schema:</p>
  
  <pre><code>{
      "$schema": "http://json-schema.org/draft-04/schema",
      "type": "object",
      "properties": {
  
          "aaa": {
              "type": "string",
              "format": "date-time"
          }
  
  
      }, "required": ["aaa"]
  }
  </code></pre>
  
  <p>The JSON gets validated by the JSON-schema. However if I change the field <code>aaa</code> to "bla" the schema does not notice that it is not a date-time any longer. </p>
  
  <p>Did I miss anything in the schema?</p>