For non-ASCII object keys, raw UTF-8 bytes end up in the signed
x-amz-meta-rgwx-source-key header. Strict S3-compatible backends
normalize non-ASCII bytes when verifying SigV4, producing a signature
mismatch -> HTTP 403, surfaced in LC as -EACCES (-13).
url_encode() the value before signing. The header is write-only,
so no decode is needed.
Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>