Fix mensaDay calculation

[Importantus]

Currently, the code for calculating the date for which the meals should be fetched looks like this:

val LocalDateTime.mensaDay: LocalDate
    get() = when {
        date.dayOfWeek == SATURDAY -> date + 2.days
        date.dayOfWeek == SUNDAY || isAfterMensaHours -> date + 1.days
        else -> date
    }

The problem is that isAfterMensaHours -> date + 1.days sets the date on friday to saturday, not the next monday.

[hoppjan]

This should definitely go into v0.0.6

Waiting on the review for #39