Support trailing checksum with no signing

[TingDaoK]

Issue #, if available:

• When you do trialing checksum, apparently, you need to have the x-amz-content-sha256 header to be set to the special value even if you are not signing the request.

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.64%. Comparing base (e373ef4) to head (dd0f49e).

Files with missing lines	Patch %	Lines
source/s3_meta_request.c	77.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   89.62%   89.64%   +0.01%     
==========================================
  Files          20       20              
  Lines        6142     6144       +2     
==========================================
+ Hits         5505     5508       +3     
+ Misses        637      636       -1     

Files with missing lines	Coverage Δ
source/s3_client.c	90.09% <ø> (ø)
source/s3_meta_request.c	91.88% <77.77%> (+0.11%)	⬆️

[graebm]

nice job shuffling around these log statements and if-checks
makes more sense now
👍

[graebm]

fix & ship