ip total length header field not stored correctly

[GoogleCodeExporter]

What steps will reproduce the problem?
>>> from dpkt.ip import IP
>>> ip = IP(src='\x01\x02\x03\x04', dst='\x05\x06\x07\x08', p=1)
>>> len(ip)
20
>>> ip.opts='foo'
>>> len(ip)
23
>>> ip2=IP(str(ip))
>>> len(ip2)
20

Original issue reported on code.google.com by stepan.k...@gmail.com on 28 Aug 2014 at 2:55