如何使用NewDecode，Golang和req * http.Request解码和映射JSON对象

Coming from PHP I'm a TOTAL Golang newb here. I read a question that is VERY similar to mine but my implementation is slightly off. Whenever I go to decode using the following code the variable msg.Username is always blank. Golang doesn't throw errors here so I know I'm missing something super small here and very important but I have no idea what it could be. I would be a very appreciative of any help and am of course expecting this to be a fist-to-foerhead epiphany. Thanks!

//The JSON I'm posting to my local server is  
//{"company_domain":"example.com", "user_name":"testu","password":"testpw"}

func Login(w http.ResponseWriter, req *http.Request) {
  type Message struct {
      CompanyDomain string
      Username      string
      Password      string
  }
  //decode
  var msg Message
  decoder := json.NewDecoder(req.Body)
  err := decoder.Decode(&msg)
  if err != nil {
      err.Error())
  }
  w.Write([]byte(msg.Username))// <- This print statement always comes in blank 
  authorize, err := models.VerifyAuth(msg.Username, msg.Password, msg.CompanyDomain, w)
  if err != nil {
      err.Error())
  }
  ...

Tag your struct fields so the decoder knows how to map the json keys to your struct.

type Message struct {
    CompanyDomain string `json:"company_domain"`
    Username      string `json:"user_name"`
    Password      string `json:"password"`
}

You just need to add json:"field_name" to your struct:

type Message struct {
    CompanyDomain string `json:"company_domain"`
    Username      string `json:"user_name"`
    Password      string `json:"password"`
}