Convert fill values correctly from negative to unsigned integers

[asinghvi17]

Positive integers are unaffected by this change anyway.

Needs tests, probably.

[meggart]

Wow, how can this be intended behavior, to me it rather sounds like the files that need this are broken. Is there a publicly available dataset that has this behavior or can you share a .zarray file where this comes up? Just out of curiosity...

[asinghvi17]

I believe I got that from https://fsspec.github.io/kerchunk/cases.html#mur-sst, the original NetCDF files have this attribute too.

[coveralls]

Pull Request Test Coverage Report for Build 12200254201

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.148%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/metadata.jl	0	1	0.0%

Totals
Change from base Build 12200200130:	0.0%
Covered Lines:	908
Relevant Lines:	1054

---

💛 - Coveralls

[lazarusA]

bump