OpenAssets::MARKER is modified every OpenAssets::Payload#to_payload calls

[Yamaguchi]

OpenAssets::MARKER is constant,so SHOULD NOT be modified

$ ./bin/console 
irb(main):005:0> OpenAssets::Payload
=> OpenAssets::Payload
irb(main):006:0> OpenAssets::MARKER
=> "OA"
irb(main):007:0> OpenAssets::Payload.new([],'').to_payload
=> "OA\u0001\u0000\u0000\u0000"
irb(main):008:0> OpenAssets::MARKER
=> "OA\u0001\u0000\u0000\u0000"
irb(main):009:0> 

[azuchi]

Thanks!