I think the cassettes weren't getting matched because of the token.

Update the kwargs merging logic to have cascading priority where 
`default_kwargs` is overridden by `open_kwargs` which is overridden
by user provided `kwargs` .
Includes a regression unit test that extends the existing simple_zarr
test to ensure that the fix for duplicate keys works.

Note that the `xarray:open_kwargs` field is a part of the
[`xarray-assets`](https://github.com/stac-extensions/xarray-assets/tree/v1.0.0)
STAC extension (in Proposal stage), xref #16

Fixes #17

* pin urllib3 in tests