ISO: Changed the incorrect 'P0S' to the correct 'PT0S'

src/Duration.cpp

@@ -292,7 +292,7 @@ std::string Duration::formatISO () const
   }
   else
   {
-    return "P0S";
+    return "PT0S";
   }
 }
 

src/Variant.cpp

@@ -1979,7 +1979,7 @@ Variant::operator std::string () const
       }
       else
       {
-        return "P0S";
+        return "PT0S";
       }
     }