Set response header content-type charset to nil

[khaapamyaki]

The twirp ruby client explicitly expects the content-type to be either "application/json" or "application/protobuf".

https://github.com/twitchtv/twirp-ruby/blob/main/lib/twirp/encoding.rb#L19-L24

This leads to following error:

=> #<Twirp::ClientResp:0x00007fdf03c1eed8 
  @data=nil, 
  @error= <Twirp::Error 
    code:internal 
    msg:"Expected response Content-Type \"application/protobuf\" but found \"application/protobuf; charset=utf-8\"" 
    meta:{}
  >
>

This also conforms with the Twirp v7 spec:

Content-Type The value should be either "application/protobuf" or "application/json" to indicate the encoding of the response message. It must match the "Content-Type" header in the request.
https://github.com/twitchtv/twirp/blob/ded153e52e9e279471e020135e42e5e3f1000276/docs/spec_v7.md#responses

This PR sets the content-type's charset to nil to explicitly remove the default utf8 charset

https://hexdocs.pm/plug/Plug.Conn.html#put_resp_content_type/3

[hernanat]

bump