Fix string type check in scipy backend
Fix string type check in scipy backend

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