replace hardcoded dash with _separator

[johlrich]

Noticed a hardcoded hyphen in the standard CategorizeEventsByStreamPath projection. Replaced it with the passed in separator.

[ysw]

It is not easy as changing this only place. The _separator in this class tells how to extract category from the stream id. You can configure it as projection arguments.

The $ce- prefix defines an index stream for a category. While I agree it is better to use a constant everywhere, changing it will break consumers of index streams that rely on this naming convention.