Single-character run duplication in update messages (with a fix)

[lmparppei]

Thank you for the actually best CRDT implementation for Swift! However, when you do single-character edits to YText, the resulting update messages are wrong.

It's caused by the following part in YStringContent:

func encode(into encoder: YUpdateEncoder, offset: Int) {
    if offset == 0 {
        encoder.writeString(self.string as String)
    } else {
        encoder.writeString(self.string.substring(to: offset))
    }
}

These offsets work for longer runs, but fail with single characters. You don't need offset == 0 check here, because we can just create a substring from the given offset, even if it's zero.

func encode(into encoder: YUpdateEncoder, offset: Int) {
    encoder.writeString(self.string.substring(from: offset))
}

I'm not sure if you are willing to maintain the project in any way, so I didn't submit a pull request yet.