[Bug] - x-amz-content-sha256 is ignored in public buckets PutObject

[niksis02]

Describe the bug
When creating an object with PutObject in public buckets by providing invalid/incorrect x-amz-content-sha256 versitygw ignores it.

To Reproduce

// create a bucket
aws s3api create-bucket --bucket test

// make the bucket public
aws s3api put-bucket-policy --bucket test --policy file://policy.json

// Attempt to put an object with invalid x-amz-content-sha256
curl -v -i -X PUT "http://localhost:7070/test/hello" -H "x-amz-content-sha256: invalid_sha256" -H "Content-Length: 100" --data-binary @data.bin

Where policy.json is:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "Statement1",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:*",
            "Resource": "arn:aws:s3:::test"
        },
        {
            "Sid": "Statement2",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:*",
            "Resource": "arn:aws:s3:::test/*"
        }
    ]
}

And data.bin is a rangom 100 length file.

Expected behavior
The gateway should return an error like:

<?xml version="1.0"?>
<Error>
  <Code>XAmzContentSHA256Mismatch</Code>
  <Message>The provided 'x-amz-content-sha256' header does not match what was computed.</Message>
  <ClientComputedContentSHA256>invalid_sha256</ClientComputedContentSHA256>
  <S3ComputedContentSHA256>aa58b21b01d6b8a99c1a5856962dbac36c758a79dc0a77c2e013ce2c39ecdc8a</S3ComputedContentSHA256>
  <RequestId>AHGMD5Z600MWF7PS</RequestId>
  <HostId>N7coEYwRhqN0U1R9LQskxDGfN9ip1uicH22LkiFVPYNomg9XbCf3SrRPVeZmq0L8OVn1H9lP3l4=</HostId>
</Error>