FixInvalidDayOffsetPreprocessor fails

[sunkup]

FixInvalidDayOffsetPreprocessor processes the whole ics file as a long string. The tests only run on a string containing the isolated DURATION property though, so they did not catch the inability to parse DURATION as a VALUE.

Reported in ICSx5: bitfireAT/icsx5#462

This works fine:

@Test
fun test_FixString_CompleteICalProperty() {
    fixAndAssert("DURATION:P1D", "DURATION:PT1D")
}

and this does not:

@Test
fun test_FixString_CompleteICalProperty() {
    fixAndAssert(
        "REFRESH-INTERVAL;VALUE=DURATION:P1D",
        "REFRESH-INTERVAL;VALUE=DURATION:PT1D"
    )
}