Issue with combination positive and negative ByMonthDay

I've been using this package for a while and think I've run into a bug (it's great - by the way).

When using both a positive and negative `BYMONTHDAY`, there will never be any results returned.

Specifically, line 423 of `RecurrenceRuleTransformer` will always evaluate true and therefore unset every day of the `$daySet`.

My use case for using a positive and negative `BYMONTHDAY` is to have twice monthly recurring items happen on the 15th and last day of the month.

For example, if a recurring charge is set for the 31st of October, I have logic to actually set it for the last day of the month going forward (so that February, April, etc will collect payments and be included in the `getComputedArray`).