Don't add NaNs for _FillValue

[matt-long]

xarray sets a _FillValue = NaN on writing to file if the _FillValue field in encoding is not set. I think this is undesirable; the preferred behavior would be for the absence of _FillValue to be preserved.

This could be addressed in get_original_attrs by including something like the following.

    if '_FillValue' not in encoding:
        encoding['_FillValue'] = None

[andersy005]

This is now fixed. Thank you for the pointer, @matt-long!