fix: update access modifier of class PrimitiveType.DateStub

Author: K5qu4r3d

modules/swagger-core/src/main/java/io/swagger/v3/core/util/PrimitiveType.java

@@ -572,7 +572,7 @@ private static <K> void addMultiKeys(Map<K, Collection<PrimitiveType>> map, Prim
         }
     }
 
-    private static class DateStub {
+    public static class DateStub {
         private DateStub() {
         }
     }