map(f,::DNDSparse) fix for changing value type

[gkappler]

trafficstars

fixes failing

filter(isodd, map(v->v.a+1,ndsparse((numid=1:10,), (a=1:10,b=1:10); chunks=2)))

[gkappler]

promote_type seemed overkill at first to me; yet it might be required in corner cases.