title: Hapi-Swagger failing with header value
body: <p>I am using hapi-swagger in our application where one of API trying to use custom header but when I ivoke that API with custom header getting below error</p>
  
  <pre><code>{
  "statusCode": 400,
  "error": "Bad Request",
  "message": "Invalid request headers input"
  }
  </code></pre>
  
  <p>Below the API where I am using headers with validator.</p>
  
  <pre><code>{
          method: 'POST',
          path: '/v1/testapi',
          config: {
              description: 'Greet user',
              notes: ['Use to greet a user'],
              tags: ['api'],    
              handler: function ( request, h ) {
                  console.log('sending response...');
                  return h.response('OK');
              },
              validate: {
                  headers: {
                      name: Joi.string().required()
                  }
              }                               
          }
      }
  </code></pre>
  
  <p>Below are the versions we are using.</p>
  
  <p>"hapi": "17.2.2", </p>
  
  <p>"hapi-swagger": "9.1.1",</p>
  
  <p>"joi": "13.1.2",</p>