fix: reject a ListBuckets bucket-region other than the gateway region

S3 never lists an empty result for another region: a valid region
other than the endpoint's own gets 400 InvalidArgument, "Requests with
bucket-region specified must be made to the corresponding regional
endpoint", with no ArgumentValue. The gateway is a single regional
endpoint, so it now does the same instead of clearing the listing.

Match the region case-insensitively like S3, and check an empty
bucket-region= too by looking for the query argument itself, since an
empty value is not a valid AWS region either.

s3err/invalid-argument.go

@@ -74,9 +74,14 @@ const (
 	InvalidArgSSEInvalidEncryptionMethod
 	InvalidArgSSECRequiresTLS
 	InvalidArgUploadIdWithUploads
+	InvalidArgBucketRegionEndpoint
 )
 
 var invalidArgErrResponses = map[InvalidArgErrorCode]InvalidArgumentError{
+	InvalidArgBucketRegionEndpoint: {
+		Description:  "Requests with bucket-region specified must be made to the corresponding regional endpoint",
+		ArgumentName: "bucket-region",
+	},
 	InvalidArgMaxBuckets: {
 		Description:  "Argument max-buckets must be an integer between 1 and 10000.",
 		ArgumentName: "max-buckets",
@@ -386,9 +391,13 @@ func GetInvalidArgObjectOwnership(value string) InvalidArgumentError {
 }
 
 func GetInvalidArgBucketRegion(value string) InvalidArgumentError {
+	description := fmt.Sprintf("Argument value %s is not a valid AWS Region", value)
+	if value == "" {
+		description = "Argument value is not a valid AWS Region"
+	}
 	return InvalidArgumentError{
 		ArgumentName: "bucket-region",
 		// no ArgumentValue is returned for this error
-		Description: fmt.Sprintf("Argument value %s is not a valid AWS Region", value),
+		Description: description,
 	}
 }