Let's use constant instead of hardcoding values (same is done in bssl)

conn.go

@@ -365,8 +365,8 @@ func (hc *halfConn) decrypt(b *block) (ok bool, prefixLen int, alertValue alert)
 				}
 				// Check AD header, see 5.2 of RFC8446
 				additionalData = make([]byte, 5)
-				additionalData[0] = 23
-				binary.BigEndian.PutUint16(additionalData[1:], 0x0303)
+				additionalData[0] = byte(recordTypeApplicationData)
+				binary.BigEndian.PutUint16(additionalData[1:], VersionTLS12)
 				binary.BigEndian.PutUint16(additionalData[3:], uint16(len(payload)))
 			}
 			var err error