Validate response headers based on specified header

Add validate_headers step in response validator.
Using JSON::Validator with validate header value with swagger header 
object.

test-app/spec/integration/blogs_spec.rb

@@ -50,6 +50,10 @@ describe 'Blogs API', type: :request, swagger_doc: 'v1/swagger.json' do
       produces 'application/json'
 
       response '200', 'blog found' do
+        header 'ETag', type: :string
+        header 'Last-Modified', type: :string
+        header 'Cache-Control', type: :string
+
         schema '$ref' => '#/definitions/blog'
 
         response_examples 'application/json' => {