Add default for most readers

src/multi/generic_multiple_barcode_reader.rs

@@ -37,6 +37,7 @@ use super::MultipleBarcodeReader;
  *
  * @author Sean Owen
  */
+#[derive(Default)]
 pub struct GenericMultipleBarcodeReader<T: Reader>(T);
 
 impl<T: Reader> MultipleBarcodeReader for GenericMultipleBarcodeReader<T> {

src/multi/qrcode/qr_code_multi_reader.rs

@@ -34,6 +34,7 @@ use super::detector::MultiDetector;
  * @author Sean Owen
  * @author Hannes Erven
  */
+#[derive(Default)]
 pub struct QRCodeMultiReader(QRCodeReader);
 impl MultipleBarcodeReader for QRCodeMultiReader {
     fn decode_multiple(