QRCodeDetector::decodeMulti() fixed invalid usage fixedType()

* fixed invalid usage fixedType()
changed default barcode type to CV_8UC1
added tests
added assert in case multi channel straight barcode input

* deleted extra wrap into OutputArray

* fix warnings

* objdetect(qr): remove unnecessary checks
Co-authored-by: NAlexander Alekhin <alexander.a.alekhin@gmail.com>