Correct some bugs on IPv6 length calculation.

* Packet#recalc now call ipv6_recalc with arg equals to :all or :ipv6.
* IPv6Header#ipv6_recalc now calls itself with arg=:ipv6_len (instead of
  :len) when called with arg=:all.
Merge branch 'master' into ipv6
Merge pull request #67 from sdaubert/ipv6

Correct some bugs on IPv6 length calculation.