Fix lib/mhc/event.rb to handle comma-separeted X-SC-Day

My mhc data which I convert by using update-uuid.sh includes comma (",") in X-SC-Day field.

So this pull request is to handle comma-separeted X-SC-Day.

I'm not sure whether I should hack mhc itself or update-uuid.sh, or should create an alternative script to replace comma of X-SC-Day in mhc data to space.