Why not just write a couple of utility functions?

utility.go

package main

import (
	"bytes"
	"encoding/json"
	"io"
	"net/http"
)

type jsonMessage struct {
	Message string `json:"msg"`
}

type jsonError struct {
	Error string `json:"error"`
}

func index(w http.ResponseWriter, r *http.Request) {
	// Return a blank error
	http.Error(w, "not found", http.StatusNotFound)

	// Return a JSON error
	serveJSON(w, jsonError{"not found"}, http.StatusNotFound)

	// Return plain text
	contentType(w, "text/plain")
	serveBytes(w, []byte("test"), http.StatusOK)

	// Return html
	contentType(w, "text/html")
	serveBytes(w, []byte("<h1>test</h1>"), http.StatusOK)

	// Return JSON data
	serveJSON(w, jsonMessage{"test"}, http.StatusOK)

	// Return a generic reader
	b := bytes.NewBufferString("test")
	contentType(w, "text/plain")
	serveReader(w, b, http.StatusOK)
}

func contentType(w http.ResponseWriter, t string) {
	w.Header().Set("Content-Type", t)
}

func serveJSON(w http.ResponseWriter, content interface{}, status int) {
	b, err := json.Marshal(content)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	w.Write(b)
}

func serveBytes(w http.ResponseWriter, content []byte, status int) {
	w.WriteHeader(status)
	w.Write(content)
}

func serveReader(w http.ResponseWriter, content io.Reader, status int) {
	w.WriteHeader(status)
	io.Copy(w, content)
}

func serveText(w http.ResponseWriter, content []byte, status int) {
	w.Header().Set("Content-Type", "text/plain")
	w.WriteHeader(status)
	w.Write(content)
}

func main() {
	http.Handle("/test", http.HandlerFunc(index))
	http.ListenAndServe(":8081", nil)
}