Fixed SkipOnUpdate

[CryptKat]

There may be properties which should be writeable only at insertion time (SkipOnInsert = false, SkipOnUpdate = true). Atm, SkipOnUpdate is mapped from BeforeSaveBehavior, which is wrong, because it will skip value not only on update, but also on insert. SkipOnUpdate must be mapped from AfterSaveBehavior. Fixed.