MINOR: sample: make the bool type cast to bin

MINOR: sample: make the bool type cast to bin

Any type except bool could cast to bin, while it can cast to string.
That's a bit inconsistent, and prevents a boolean from being used as
the entry of a hash function while any other type can. This is a
problem when passing via variable where someone could use:

    ... set-var(txn.bar) always_false

to temporarily disable something, but this would result in an empty
hash output when later doing:

    ... var(txn.bar),sdbm

Instead of using c_int2bin() as is done for the string output, better
enfore an set of inputs or exactly 0 or 1 so that a poorly written sample
fetch function does not result in a difficult to debug hash output.