fixed "format: date" serialization

Author: a-ignatov-parc

modules/openapi-generator/src/main/resources/typescript/model/ObjectSerializer.mustache

@@ -152,12 +152,7 @@ export class ObjectSerializer {
             return transformedData;
         } else if (type === "Date") {
             if (format == "date") {
-                let month = data.getMonth()+1
-                month = month < 10 ? "0" + month.toString() : month.toString()
-                let day = data.getDate();
-                day = day < 10 ? "0" + day.toString() : day.toString();
-
-                return data.getFullYear() + "-" + month + "-" + day;
+                return data.toISOString().substring(0,10);
             } else {
                 return data.toISOString();
             }