Add missing upstream unit tests for httpchecksum

[jakob-keller]

Description of Change

Add missing upstream unit tests for httpchecksum

Assumptions

Patched codepaths should be covered by unit tests.

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 6 lines in your changes missing coverage. Please review.

Project coverage is 89.84%. Comparing base (7c41f8a) to head (8d716f1).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
tests/boto_tests/unit/test_httpchecksum.py	97.14%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1294      +/-   ##
==========================================
+ Coverage   88.69%   89.84%   +1.14%     
==========================================
  Files          67       67              
  Lines        5875     6083     +208     
==========================================
+ Hits         5211     5465     +254     
+ Misses        664      618      -46     

Flag	Coverage Δ
unittests	89.84% <97.14%> (+1.14%)	⬆️

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