Add workaround to prevent object array

Add a workaround for pydata/xarray#1792 ---
the mask was accidentally being turned into an object dtype array.

Author: gerritholl

fogpy/composites.py

@@ -279,9 +279,9 @@ def _convert_to_xr(projectables, fls, mask):
                 "start_time", "end_time", "area", "resolution")}
 
         xrfls = xarray.DataArray(
-                fls, dims=dims, coords=coords, attrs=attrs)
+                fls.data, dims=dims, coords=coords, attrs=attrs)
         xrmsk = xarray.DataArray(
-                mask, dims=dims, coords=coords, attrs=attrs)
+                mask.data, dims=dims, coords=coords, attrs=attrs)
 
         return (xrfls, xrmsk)