Packet().overload_fields can be overwritten

[p-l-]

This is needed by fragment(), which has been broken since PR #11.

[p-l-]

Before:

>>> fragment(IP() / ("X" * 10), 8)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "scapy/layers/inet.py", line 766, in fragment
    r.overload_fields = p[IP].payload.overload_fields.copy()
  File "scapy/packet.py", line 217, in __setattr__
    except AttributeError:
AttributeError: 'Raw' object attribute 'overload_fields' is read-only

After:

>>> fragment(IP() / ("X" * 10), 8)
[<IP  flags=MF frag=0 |<Raw  load='XXXXXXXX' |>>, <IP  flags= frag=1 |<Raw  load='XX' |>>]

[guedou]

I think that it would be a good idea to addd this call to fragment to the unit tests.

[p-l-]

@guedou agreed! Plus that allowed me to notice that defragment() was broken, too :)

[guedou]

I guess that this is a debugging statement that was forgotten =)

[p-l-]

Hum. Thanks :)