A custom Advertisement with a constructor can fail to parse incoming BLE data correctly

This innocent looking constructor in a sub-class of `Advertisement`:

```
def __init__(self, enc_data=None, round=0):
super().__init__()
if enc_data is not None:
self.enc_data = enc_data
if round is not None:
self.round = round
```

was causing ads to be receieved on another Adafruit device use this library as

```
>
```

Minor possibility I've misdiagnosed this and they are sent like this due to a bug but this seems unlikely.

Originally mentioned in the comments of https://github.com/adafruit/Adafruit_CircuitPython_BLE/issues/79#issuecomment-634990957

Start with the custom Advertisement constructor shown in the issue and trace how incoming BLE data becomes the displayed manufacturer_data. Reproduce the reported parsing result on an Adafruit device if possible, then verify that advertisements received through a subclass constructor retain the expected data.

Written by the indexing model from the issue text.