Fixes response_validator to handle 3.0 responses and validate against the schema.

JSON::Validator already handles anyOf oneOf schema definitions, so those can be passed in and validation errors are returned properly

test-app/spec/integration/blogs_spec.rb

@@ -74,7 +74,7 @@ describe 'Blogs API', type: :request, swagger_doc: 'v1/swagger.json' do
       let(:flexible_blog) { { blog: { headline: 'my headline', text: 'my text' } } }
 
       response '201', 'flexible blog created' do
-        schema '$ref' => '#/components/schemas/blog'
+        schema :oneOf => [{'$ref' => '#/components/schemas/blog'},{'$ref' => '#/components/schemas/flexible_blog'}]
         run_test!
       end
     end
@@ -120,7 +120,7 @@ describe 'Blogs API', type: :request, swagger_doc: 'v1/swagger.json' do
     end
   end
 
-  
+
   path '/blogs/{id}/upload' do
     let(:id) { blog.id }
     let(:blog) { Blog.create(title: 'foo', content: 'bar') }

test-app/spec/swagger_helper.rb

@@ -54,9 +54,9 @@ RSpec.configure do |config|
               id: { type: 'integer' },
               title: { type: 'string' },
               content: { type: 'string', nullable: true },
-              thumbnail: { type: 'string' }
+              thumbnail: { type: 'string', nullable: true }
             },
-            required: %w[id title content thumbnail]
+            required: %w[id title]
           },
           flexible_blog: {
               type: 'object',
@@ -64,9 +64,9 @@ RSpec.configure do |config|
                   id: { type: 'integer' },
                   headline: { type: 'string' },
                   text: { type: 'string', nullable: true },
-                  thumbnail: { type: 'string' }
+                  thumbnail: { type: 'string', nullable:true }
               },
-              required: %w[id headline thumbnail]
+              required: %w[id headline]
           }
         },
         securitySchemes: {