Fix HeadObject body headers

## Linked issue Fixes #450. ## Problem `Bucket::head_object` builds its request through the common request header path. The generic fallback branch added `Content-Length: 0` and `Content-Type: text/plain` for commands that are not explicitly exempted. `GetObject` was exempted, but `HeadObject` was not, so HEAD requests emitted and signed unnecessary body headers. ## Solution Add `Command::HeadObject` to the same narrow no-body-header branch used by `GetObject`/listing-style commands. This avoids PR #452's broader `has_body()` style change and leaves upload, multipart initiation/completion, GCS-related multipart paths, and other body-bearing commands untouched. ## Focused verification - `cargo test -p rust-s3 --no-default-features --features sync head_object_omits_body_headers_from_request_and_signature -- --nocapture` - Result: passed; 1 passed, 0 failed. - `cargo test -p rust-s3 --no-default-features --features sync request::blocking::tests -- --nocapture` - Result: failed in two pre-existing path-style host assertions; the new `head_object_omits_body_headers_from_request_and_signature` test passed. The failures assert path-style host behavior and are unrelated to HeadObject/body headers. ## Risk notes - Signing risk: intentionally changes the HeadObject canonical signed header set by removing `content-length` and `content-type`; regression coverage asserts those headers are absent both from the request and from `SignedHeaders` while preserving `x-amz-content-sha256`. - Provider/runtime cordons: change is in the shared request trait header construction, so it applies across sync/tokio/async-std backends. No URL construction, credential resolution, security token handling, request payload SHA256, multipart upload, or GCS-specific multipart behavior was changed. <!-- Reviewable:start --> - - - This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/durch/rust-s3/455) <!-- Reviewable:end --> <!-- This is an auto-generated comment: release notes by coderabbit.ai --> ## Summary by CodeRabbit * **Bug Fixes** * HEAD Object requests now correctly exclude Content-Length and Content-Type from both the sent request and the signature, improving API compliance. * **Tests** * Added a unit test that verifies HEAD Object requests omit body-related headers from request headers and the signed headers list. <!-- end of auto-generated comment: release notes by coderabbit.ai -->