Why can't we make a good comparison? ruby-ffi call golang(so files)

I'm calling shared files created by Golang from ruby-ffi.
However, when I run Golang alone, the process works, but when I run it via ruby-ffi, the comparison doesn't work.

package main

import (
  "C"
  "fmt"
)

//export zen_roma_to_han_roma
func zen_roma_to_han_roma(s string) string {
  if s=="test" {
    return "10"
  }
  return s
}

func main(){
  fmt.Println(zen_roma_to_han_roma("test"))
}

$ go run replace.go                                                                                                                                                               
10

Here's an example of what can go wrong.
First, create a go build and load it.

go build -buildmode=c-shared -o replace.so replace.go     

module Sample
  extend FFI::Library
  ffi_lib 'replace.so'

  attach_function :zen_roma_to_han_roma, [:string], :string
end

[5] pry(main)> Sample.zen_roma_to_han_roma("test")
=> "test

I don't know the cause of this and I need your help.
Is there anything else I should try?

I am not a Golang programmer but still here is a quote from the FFI gem documentation

> :string should be considered to be const char * and the Ruby string must not be changed as long as it’s accessed by the library. If the string buffer shall be modified from C or Ruby side, use :pointer and FFI::MemoryPointer instead.

So you should pass *C.char as an argument and return a value of the same type instead of string.

package main

import (
  "C"
  "fmt"
)

//export zen_roma_to_han_roma
func zen_roma_to_han_roma(s *C.char) *C.char {
  test_string := C.GoString(s)
  if test_string == "test" {
    return C.CString("10")
  }
  return s
}

func main() {
   fmt.Println(C.GoString(zen_roma_to_han_roma(C.CString("test"))))
}