Retain original _FillValue in encoding

[maxrjones]

This is a follow-up to #471 that adds _FillValue to the retained encoding options. Based on #471 (comment), _FillValue was initially excluded because it causes the tests to fail when asserting that the expected and actual schemas are equal. That fails because it's common to specify _FillValue as np.nan and nans do not equal each other. This PR adds a _assert_schema_equal that will treat NaNs in the schema as equal, so that the original _FillValue can be retained.

This PR is a prerequisite to #669 because we need to make all encoding options stored in .zmetadata follow valid JSON.

[cisaacstern]

Thanks @maxrjones! Based on discussion following #471 (comment), it does not seem as though there was a specific justification for excluding _FillValue initially, aside from the testing issues resolved by this PR. This track with what you shared in the meeting today! So in principle, this is good by me. I'll just wait to make sure tests pass.

[cisaacstern]

Thanks! @norlandrhagen please feel free to release per https://pangeo-forge.readthedocs.io/en/latest/contributing.html#releasing if it's useful for you all!

[maxrjones]

thanks for the prompt reviews @cisaacstern and @norlandrhagen!