add test if dataset has a dimension without coordinates
add test if dataset has a dimension without coordinates

jhamman/xarray,rabernat/xray,jcmgray/xarray,shoyer/xray,jhamman/xray,shoyer/xarray,rabernat/xray,jhamman/xarray,pydata/xarray,markelg/xray,xray/xray,chunweiyuan/xarray,shoyer/xarray,jhamman/xarray,pydata/xarray,pydata/xarray,markelg/xray,markelg/xray