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/app/controllers/blogs_controller.rb

@@ -17,6 +17,10 @@ class BlogsController < ApplicationController
   # GET /blogs/1
   def show
     @blog = Blog.find_by_id(params[:id])
+
+    fresh_when(@blog)
+    return unless stale?(@blog)
+
     respond_with @blog, status: :not_found and return unless @blog
     respond_with @blog
   end

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' => {

test-app/swagger/v1/swagger.json

@@ -89,8 +89,26 @@
         "responses": {
           "200": {
             "description": "blog found",
+            "headers": {
+              "ETag": {
+                "type": "string"
+              },
+              "Last-Modified": {
+                "type": "string"
+              },
+              "Cache-Control": {
+                "type": "string"
+              }
+            },
             "schema": {
               "$ref": "#/definitions/blog"
+            },
+            "examples": {
+              "application/json": {
+                "id": 1,
+                "title": "Hello world!",
+                "content": "Hello world and hello universe. Thank you all very much!!!"
+              }
             }
           },
           "404": {