Use ASCII-8BIT for comparing pcap magic number

In Ruby 2, source files are read as UTF-8 by default.  When comparing
PCAP headers, we should use ASCII-8BIT or else the comparison will not
work.  This should be backwards compatible with Ruby 1.9.

MSP-12092

Author: bturner-r7

lib/msf/core/db_manager/import.rb

@@ -119,7 +119,7 @@ def import_file(args={}, &block)
         # Msf::DBManager::Import::MetasploitFramework::Zip becomes resolvable as Zip here, so need to use ::Zip so Zip
         # is resolved as one from rubyzip gem.
         data = ::Zip::File.open(filename)
-      when "\xd4\xc3\xb2\xa1", "\xa1\xb2\xc3\xd4"
+      when "\xd4\xc3\xb2\xa1".force_encoding('ASCII-8BIT'), "\xa1\xb2\xc3\xd4".force_encoding('ASCII-8BIT')
         data = PacketFu::PcapFile.new(:filename => filename)
       else
         ::File.open(filename, 'rb') do |f|