Fix iscompressed in zarr extension

[meggart]

Supersedes #58

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 52.04%. Comparing base (2e561cb) to head (7b381a8).

Files with missing lines	Patch %	Lines
ext/ZarrExt.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   51.84%   52.04%   +0.19%     
==========================================
  Files          12       12              
  Lines         488      490       +2     
==========================================
+ Hits          253      255       +2     
  Misses        235      235              

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