bug: timer.cancel(anId) always returns true

[philj404]

I tried creating an automated regression test to verify good behavior for timer.cancel() and timer ID collisions.
... and I think I found a regression for the returned value from timer.cancel().

    bool
    cancel(Task &task)
    {
        struct task * const t = static_cast<struct task * const>(task);

        if (t) {
            remove(t);
            task = static_cast<Task>(NULL);
            return true;
        }

        return false;
    }

Here's the regression: Now that a timer ID is just a pointer to the timer's slot in the array, it will never be 0... and so timer.cancel(anId) always returns true.

See #79 for the regression test and a fix.

[philj404]

Thanks for the merge of #79. It appears the updated unit test now PASSES.