SNOW-1569290 Make Azure headers case insensitive

[sfc-gh-pfus]

Description

SNOW-1569290 Azure client treated headers as case sensitive before, which causes fails when downloading files uploaded by other drivers.

Checklist

• Created tests which fail without the change (if possible)
• Extended the README / documentation, if necessary

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.58%. Comparing base (27e76c9) to head (a6bf5f7).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1238      +/-   ##
==========================================
- Coverage   82.59%   82.58%   -0.01%     
==========================================
  Files          55       55              
  Lines       13315    13321       +6     
==========================================
+ Hits        10997    11001       +4     
- Misses       2318     2320       +2     

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