Update pcap.rb

Fixes problem with crash when attempting to change encoding on frozen string from hash key

As written PCapFile.array_to_file will attempt to use the key part of a Hash as a binary timestamp value if the array elements passed through the method's :array/:arr/:a parameter are hashes instead of Packets. Hash keys are "frozen" in ruby 2, so an attempt to force binary encoded on a frozen string fails.

"dup"ing the hash key before attempting to force encoding solves this.