Change call to assign_coords to work on old xarray

src/metpy/xarray.py

@@ -484,7 +484,7 @@ def _has_coord(coord_type):
         coords = dict(self._rebuild_coords(var, crs))
         if crs is not None:
             coords['crs'] = crs
-        return var.assign_coords(coords)
+        return var.assign_coords(**coords)
 
     def _rebuild_coords(self, var, crs):
         """Clean up the units on the coordinate variables."""