check for the S4 flag

[moodymudskipper]

Any object can be an S4 in the sense of isS4() after it's passed to asS4(), waldo doesn't seem to check for this, if it did it would make it easier to spot corrupted objects.

isS4(asS4(1))
#> [1] TRUE
waldo::compare(1, asS4(1))
#> ✔ No differences

^{Created on 2024-03-18 with reprex v2.0.2}