Fixes#2052Fixes#2056Fixes#2057
Previously, GetObject and HeadObject used the request's `Range` header to determine the response status code, which caused incorrect 206 responses for invalid Range header values.
The status is now driven by whether res.ContentRange is set in the response, rather than by the presence of a range in the request. Backends (posix and azure) now set Content-Range for PartNumber=1 on non-multipart objects, skipping zero-size objects where no range applies.
HeadObject was also fixed to return 206 when Content-Range is present, and to only return checksums when the full object is requested.