feat: streamlined booking number property

tests/BusProNet/XmlParser/BookingResponseParserTest.php

@@ -38,7 +38,7 @@ class BookingResponseParserTest extends TestCase
         $response = $this->parser->parse($crawler->filter('ergebnis'));
 
         $this->assertTrue($response->isInquiryValid());
-        $this->assertEquals('321530', $response->transactionNumber);
+        $this->assertEquals(321530, $response->bookingNumber);
         $this->assertEquals(755.0, $response->totalPrice);
 
         // Verify purchase voucher is parsed