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.

rswag-specs/lib/rswag/specs/response_validator.rb

@@ -13,6 +13,7 @@ module Rswag
 
       def validate!(response)
         validate_code!(response.code)
+        validate_headers!(response.headers)
         validate_body!(response.body)
       end
 
@@ -24,6 +25,20 @@ module Rswag
         end
       end
 
+      def validate_headers!(headers)
+        header_schema = @api_metadata[:response][:headers]
+        return if header_schema.nil?
+        header_schema.each do |header_name, schema|
+          validate_header!(schema, header_name, headers[header_name.to_s])
+        end
+      end
+
+      def validate_header!(schema, header_name, header_value)
+        JSON::Validator.validate!(schema.merge(@global_metadata), header_value.to_json)
+      rescue JSON::Schema::ValidationError => ex
+        raise UnexpectedResponse, "Expected response headers #{header_name} to match schema: #{ex.message}"
+      end
+
       def validate_body!(body)
         response_schema = @api_metadata[:response][:schema]
         return if response_schema.nil?
@@ -34,7 +49,7 @@ module Rswag
             .merge(@global_metadata.slice(:definitions))
           JSON::Validator.validate!(validation_schema, body)
         rescue JSON::Schema::ValidationError => ex
-          raise UnexpectedResponse, "Expected response body to match schema: #{ex.message}" 
+          raise UnexpectedResponse, "Expected response body to match schema: #{ex.message}"
         end
       end
     end

rswag-specs/spec/rswag/specs/response_validator_spec.rb

@@ -66,6 +66,43 @@ module Rswag
             it { expect { call }.to raise_error UnexpectedResponse }
           end
         end
+
+        context "'headers' provided" do
+          before do
+            api_metadata[:response][:headers] = {
+              'X-Rate-Limit-Limit' => {
+                description: 'The number of allowed requests in the current period',
+                type: 'integer'
+              },
+              'X-Rate-Limit-Remaining' => {
+                description: 'The number of remaining requests in the current period',
+                type: 'integer'
+              },
+              'X-Rate-Limit-Reset' => {
+                description: 'The number of seconds left in the current period',
+                type: 'integer'
+              }
+            }
+          end
+
+          context 'response code & body matches' do
+            let(:response) { OpenStruct.new(code: 200, body: '{}', headers: {
+              'X-Rate-Limit-Limit' => 1,
+              'X-Rate-Limit-Remaining' => 1,
+              'X-Rate-Limit-Reset' => 1
+            }) }
+            it { expect { call }.to_not raise_error }
+          end
+
+          context 'response code matches & body does not' do
+            let(:response) { OpenStruct.new(code: 200, body: '{}', headers: {
+              'X-Rate-Limit-Limit' => 'invalid',
+              'X-Rate-Limit-Remaining' => 'invalid',
+              'X-Rate-Limit-Reset' => 'invalid'
+            }) }
+            it { expect { call }.to raise_error UnexpectedResponse }
+          end
+        end
       end
     end
   end