in lib/src/util.dart [124:134]
List<String> expandHeaderValue(Object v) {
if (v is String) {
return [v];
} else if (v is List<String>) {
return v;
} else if ((v as dynamic) == null) {
return const [];
} else {
throw ArgumentError('Expected String or List<String>, got: `$v`.');
}
}