Use `Packet().raw_packet_cache` attribute when iterating

When iterating over `Packet` instances, `.raw_packet_cache` attribute
was not used, causing bugs similar to the one detailed in issue #913
to appear when packets sniffed or built from strings representations
were used with `wrpcap()`, `send`(`p`?), `sr`(`p`?), etc.

This commit essentially copies the `.raw_packet_cache` attribute in
`.clone_with()` method.

--HG--
branch : issue-5105