bugfix: unmasking reinjected the new (i.e. nan) instead of the old values (this should have been caught by a test)

function, as dynamically assigned variables are missing otherwise

        failed, as they were evaluated during 'compile' time instead
        of execution time

refactored drop_flags to to_drop as former implies a boolen value, but a array-like or scalar is expected.

The tests wasn't included in the last merged merge-request !52 (masking b),
because it was a complete rewrite from scratch, because !45 was outdated.