acl: Support PUT calls with success for 'private' ACL's (#9000)

Add dummy calls which respond success when ACL's
are set to be private and fails, if user tries
to change them from their default 'private'

Some applications such as nuxeo may have an
unnecessary requirement for this operation,
we support this anyways such that don't have
to fully implement the functionality just that
we can respond with success for default ACLs

cmd/auth-handler.go

@@ -380,12 +380,11 @@ func isReqAuthenticated(ctx context.Context, r *http.Request, region string, sty
 		err                       error
 		contentMD5, contentSHA256 []byte
 	)
+
 	// Extract 'Content-Md5' if present.
-	if _, ok := r.Header[xhttp.ContentMD5]; ok {
-		contentMD5, err = base64.StdEncoding.Strict().DecodeString(r.Header.Get(xhttp.ContentMD5))
-		if err != nil || len(contentMD5) == 0 {
-			return ErrInvalidDigest
-		}
+	contentMD5, err = checkValidMD5(r.Header)
+	if err != nil {
+		return ErrInvalidDigest
 	}
 
 	// Extract either 'X-Amz-Content-Sha256' header or 'X-Amz-Content-Sha256' query parameter (if V4 presigned)