Prevent golang http.NewRequest from adding braces to POST body

This must be fairly simple, but I can't work out why, when making an HTTP request with go, the body of the request gets wrapped in an additional set of braces:

package main

import (
    "bytes"
    "fmt"
    "net/http"
)

func main() {
    jsonStr := []byte(`{"some":"test","json":"data"}`)
    req, _ := http.NewRequest("POST", "http://test.com", bytes.NewBuffer(jsonStr))
    fmt.Print(req.Body)
}

This results in:

{{"some":"test","json":"data"}}

In my actual code I'm using the json.Marshal and a struct to generate the byte buffer, but getting the same result. The result is the API rejecting the request (as expected).

How do I prevent the extra braces being added?