Don't wrap at non breaking space.

[u0nel]

unicode.IsSpace also returns true for a non breaking space (0xA0), which isn't supposed to be treated as a space in this context.

https://github.com/muesli/reflow/blob/00a9f5c6902562434539e11d2c8f8d3dae851318/wordwrap/wordwrap.go#L127