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