Support NaN and Inf user attributes

[constantinpape]

A user specified attribute that is nan or inf will cause issues because it's not properly decoded / encoded in json. This issue is also present in zarr-python, see https://github.com/zarr-developers/zarr-python/issues/412. I have proposed a fix in https://github.com/zarr-developers/zarr-python/pull/533. If this gets merged, we can just apply the same solution as for the fill value (#149 ) for all json attributes.