How to use raw header with evans cli interface

[anexus7]

using evans --header a="b,c" creates headers as :

Header: config.Header{
      "grpc-client": []string{
        "evans",
      },
      "a": []string{
        "b",
        "c",
      },
    }

to get header like a="b,c" is there any way to parse it as a raw string ?
I've tried using escape character as well like a="b,c" but it still doesn't work.

It is possible in the REPL way as the header command there has a --raw flag.
Any help would be appreciated, thanks!