destiny: change byte order of ipv6_header->length
When accessing the length field of an ipv6_header a byte order switch (host -> network) is necessary.
Otherwise, it breaks calculations or the checksum and other tcp related computations.
Furthermore, when writing to ipv6_header->length it is important to switch this
from host byte order to network byte order.
Merge pull request #1300 from cgundogan/fix_wrong_byte_order_ipv6_length

destiny: change byte order of ipv6_header->length