add content-sha256 header for presign when the signed_body_value is set to UNSIGNED-PAYLOAD

[TingDaoK]

Issue #, if available:

• When signing via query-params, there is no available way to sign content-sha256 header

Description of changes:

• When the signed_body_value is set to UNSIGNED-PAYLOAD, signer will add the content-sha256 header even signing via query-params.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 80.40%. Comparing base (877c029) to head (bb68463).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #248   +/-   ##
=======================================
  Coverage   80.39%   80.40%           
=======================================
  Files          33       33           
  Lines        6035     6038    +3     
=======================================
+ Hits         4852     4855    +3     
  Misses       1183     1183           

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