s3api: map truncated SigV4 headers to the error for the missing field

AWS answers an Authorization header missing Credential= with
InvalidArgument and one missing or malformed Signature= with
AuthorizationHeaderMalformed, instead of a generic MissingFields.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/s3api/s3err/s3api_errors.go

@@ -486,7 +486,7 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		HTTPStatusCode: http.StatusBadRequest,
 	},
 	ErrMissingSignTag: {
-		Code:           "AccessDenied",
+		Code:           "AuthorizationHeaderMalformed",
 		Description:    "Signature header missing Signature field.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},