Return true instead of nil when validation passes

As mentioned in #101 it makes more sense to return true from this method
when validation passes instead of nil.

lib/openapi_parser/schemas/response.rb

@@ -25,7 +25,7 @@ def validate(response_body, response_validate_options)
           raise ::OpenAPIParser::NotExistContentTypeDefinition, object_reference
         end
 
-        return nil
+        return true
       end
 
       options = ::OpenAPIParser::SchemaValidator::Options.new # response validator not support any options