title: Validation for required boolean attributes doesn't work
body: <p>I'm using OpenAPI 2.0 contract for my Rest service, written in Yaml, and I'm using swagger-codegen to generate the POJOs. I activated the useBeanValidation and performBeanValidation, so that the generated code (in JAVA) include the validation annotations...</p>
  
  <p>In my contract I have a field of type boolean that I marked as required:</p>
  
  <pre><code>cancellationInd:
          description: "indicates if this is a cancellation"
          type: boolean
          required: true
  </code></pre>
  
  <p>and the generated code is as follow:</p>
  
  <pre><code>/**
     * indicates if this is a cancellation.
     * @return cancellationInd
    **/
    @NotNull
    @ApiModelProperty(required = true, value = "indicates if this is a cancellation")
    public Boolean isCancellationInd() {
      return cancellationInd;
    }
  
    public void setCancellationInd(Boolean cancellationInd) {
      this.cancellationInd= cancellationInd;
    }
  </code></pre>
  
  <p>The problem is that in my JsonREquest to the serviec, when I used
  cancellationInd: null, it passes.... I was expecting some HttpStatus_400
  because of the required field. Same thing when field is emtpy instead of null...</p>
  
  <pre><code>{
  ...
    "timstamp": "2019-04-29T13:23:56.123-04:00",
    "cancellationInd": "false",
  ...
  
  }  
  </code></pre>
  
  <p>It looks like the reason is that when the attribute is of type boolean, the generated  one in java is Boolean (which is a wrapper, not primitive, and thus accepts a null)... am I right? Is this is a bug?</p>
  
  <p>Any suggestions please?</p>