Add unit test to check that multiple mising values issues warning
Add unit test to check that multiple mising values issues warning

jhamman/xray,chunweiyuan/xarray,cpaulik/xray,rabernat/xray,drewokane/xray,NicWayand/xray,shoyer/xarray,jhamman/xarray,shoyer/xray,cpaulik/xray,petercable/xray,markelg/xray,shoyer/xarray,markelg/xray,markelg/xray,kjordahl/xray,xray/xray,rabernat/xray,jhamman/xarray,pydata/xarray,jhamman/xarray,jcmgray/xarray,kjordahl/xray,pydata/xarray,pydata/xarray