Add test for existing _FillValue in encoding

[maxrjones]

In order to ensure compliance with the JSON spec and readability by zarr-js, we clear existing encodings in the datatree and set the _FillValue for each variable using the set_zarr_encoding utility function. While this is generally good, I am concerned that this could lead to the misinterpretation of data if the dataset already had a different no data value set via _FillValue and mask_and_scale was not used when loading the dataset. It seems worthwhile to add a test for that case.