Regression with UNSIGNED-PAYLOAD requests

Following https://review.opendev.org/c/openstack/swift/+/767644, UNSIGNED-PAYLOAD requests all fail with SignatureDoesNotMatch errors. The trouble is the lower-casing of the provided X-Amz-Content-SHA256 value; the server then calculates a canonical request like

========================================
PUT
/bucket/server.py

content-md5:KRI0BANRt8GOkkAnWigz4w==
content-type:text/x-python
host:saio
x-amz-content-sha256:UNSIGNED-PAYLOAD
x-amz-date:20210108T224842Z

content-md5;content-type;host;x-amz-content-sha256;x-amz-date
unsigned-payload
========================================

while the client calculated

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PUT
/bucket/server.py

content-md5:KRI0BANRt8GOkkAnWigz4w==
content-type:text/x-python
host:saio
x-amz-content-sha256:UNSIGNED-PAYLOAD
x-amz-date:20210108T224842Z

content-md5;content-type;host;x-amz-content-sha256;x-amz-date
UNSIGNED-PAYLOAD
========================================