Fix GrantNewAccessTokenFromAuthCode method.

Changes made:
1. Body is sent as a form-urlencoded values instead of json
2. Basic auth is used for authorization instead of an access token

client.go

@@ -37,11 +37,10 @@ func (c *Client) GetAccessToken() (*TokenResponse, error) {
 		return &TokenResponse{}, err
 	}
 
-	req.SetBasicAuth(c.ClientID, c.Secret)
 	req.Header.Set("Content-type", "application/x-www-form-urlencoded")
 
 	t := TokenResponse{}
-	err = c.Send(req, &t)
+	err = c.SendWithBasicAuth(req, &t)
 
 	// Set Token fur current Client
 	if t.Token != "" {
@@ -146,6 +145,13 @@ func (c *Client) SendWithAuth(req *http.Request, v interface{}) error {
 	return c.Send(req, v)
 }
 
+// SendWithBasicAuth makes a request to the API using clientID:secret basic auth
+func (c *Client) SendWithBasicAuth(req *http.Request, v interface{}) error {
+	req.SetBasicAuth(c.ClientID, c.Secret)
+
+	return c.Send(req, v)
+}
+
 // NewRequest constructs a request
 // Convert payload to a JSON
 func (c *Client) NewRequest(method, url string, payload interface{}) (*http.Request, error) {

identity.go

@@ -3,25 +3,28 @@ package paypalsdk
 import (
 	"fmt"
 	"net/http"
+	"net/url"
+	"strings"
 )
 
 // GrantNewAccessTokenFromAuthCode - Use this call to grant a new access token, using the previously obtained authorization code.
 // Endpoint: POST /v1/identity/openidconnect/tokenservice
 func (c *Client) GrantNewAccessTokenFromAuthCode(code string, redirectURI string) (*TokenResponse, error) {
-	type request struct {
-		GrantType   string `json:"grant_type"`
-		Code        string `json:"code"`
-		RedirectURI string `json:"redirect_uri"`
-	}
-
 	token := &TokenResponse{}
 
-	req, err := c.NewRequest("POST", fmt.Sprintf("%s%s", c.APIBase, "/v1/identity/openidconnect/tokenservice"), request{GrantType: "authorization_code", Code: code, RedirectURI: redirectURI})
+	q := url.Values{}
+	q.Set("grant_type", "authorization_code")
+	q.Set("code", code)
+	q.Set("redirect_uri", redirectURI)
+
+	req, err := http.NewRequest("POST", fmt.Sprintf("%s%s", c.APIBase, "/v1/identity/openidconnect/tokenservice"), strings.NewReader(q.Encode()))
 	if err != nil {
 		return token, err
 	}
 
-	err = c.SendWithAuth(req, token)
+	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+
+	err = c.SendWithBasicAuth(req, token)
 	if err != nil {
 		return token, err
 	}