Serializing openLeft and openRight

looking at the steps created while typing almost all of them look like:

from: … slice … content … o: … openLeft: 0 openRight: 0 stepType: "replace" to: …

That is they all have openLeft: 0 and openRight: 0 which is redundant space on the DB

Can we have the toJson of slice drop openLeft and openRight if they are 0 and the fromJson set them to 0 if they are missing ?

Thx!

Done in this patch