Golang: http.NewRequest POST with JSON payload returns error 500

I'm working on a library for an API. There's an API endpoint (POST) which when you issue a curl command is:

curl -H "X-API-TOKEN: API-TOKEN" 'http://interest-graph.getprismatic.com/text/topic' \
  --data "title=Clojure" \
  --data "body=Clojure is a dynamic programming language that targets the Java Virtual Machine (and the CLR, and JavaScript). It is designed to be a general-purpose language, combining the approachability and interactive development of a scripting language with an efficient and robust infrastructure for multithreaded programming.Clojure is a compiled language - it compiles directly to JVM bytecode, yet remains completely dynamic. Every feature supported by Clojure is supported at runtime."

I'm trying to perform the above command using http.NewRequest:

   var jsonReq = []byte(`{"title": "Clojure", "body": "Clojure is a dynamic programming language that targets the Java Virtual Machine 
    	(and the CLR, and JavaScript). It is designed to be a general-purpose language, 
    	combining the approachability and interactive development of a 
    	scripting language with an efficient and robust infrastructure for multithreaded programming.
    	Clojure is a compiled language - it compiles directly to JVM bytecode, 
    	yet remains completely dynamic. Every feature supported by Clojure is supported at runtime."}`)

	buf := new(bytes.Buffer)

	err := json.NewEncoder(buf).Encode(jsonReq)
	if err != nil {
		fmt.Println(err)
	}

	u := "http://interest-graph.getprismatic.com/text/topic"
	ApiToken := "API-TOKEN"
	req, err := http.NewRequest("POST", u, buf)
	req.Header.Set("Content-Type", "application/json")
	req.Header.Set("X-API-TOKEN", ApiToken)

	if err != nil {
		log.Fatal(err)
	}
	c := http.DefaultClient
	resp, err := c.Do(req)
	if err != nil {
		log.Fatal(err)
	}

	defer resp.Body.Close()

	r, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(string(r))

Not sure if I'm doing this right, because I keep getting an error 500. Any idea what I'm doing wrong?

The correct code:

package main

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"net/url"
)

func main() {

	u := "http://interest-graph.getprismatic.com/text/topic"
	ApiToken := "API-CODE"

	data := url.Values{}
	data.Set("title", "Clojure")
	data.Set("body", `Clojure is a dynamic programming language that targets the Java Virtual Machine 
    	(and the CLR, and JavaScript). It is designed to be a general-purpose language, 
    	combining the approachability and interactive development of a 
    	scripting language with an efficient and robust infrastructure for multithreaded programming.
    	Clojure is a compiled language - it compiles directly to JVM bytecode, 
    	yet remains completely dynamic. Every feature supported by Clojure is supported at runtime.`)

	b := bytes.NewBufferString(data.Encode())

	req, err := http.NewRequest("POST", u, b)
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
	req.Header.Set("X-API-TOKEN", ApiToken)

	if err != nil {
		log.Fatal(err)
	}
	c := http.DefaultClient
	resp, err := c.Do(req)
	if err != nil {
		log.Fatal(err)
	}

	defer resp.Body.Close()

	r, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(string(r))

}