Net::HTTPHeader#set_field

# File tmp/rubies/ruby-3.3.0/lib/net/http/header.rb, line 270
        def set_field(key, val)
  case val
  when Enumerable
    ary = []
    append_field_value(ary, val)
    @header[key.downcase.to_s] = ary
  else
    val = val.to_s # for compatibility use to_s instead of to_str
    if val.b.count("\r\n") > 0
      raise ArgumentError, 'header field value cannot include CR/LF'
    end
    @header[key.downcase.to_s] = [val]
  end
end