fix: missing isoDayOfWeek

[alfonsocv12]

Reading to changes on PR 472 I thinks this were missing

[dkhalanskyjb]

I'd prefer to keep the internal variable named isoDayOfWeek: it's a numerical value, and there are several ways of assigning numbers to days, so it's nice to have it unambiguous.
The name of the dayOfWeek variable in the date container does get exposed to the user, so we are limited in what we can call it.