Properly pass nil body to http.NewRequest

The integration tests revealed that AnimeService.List and
MangaService.List started receiving a 411 Length required error from the
MyAnimeList API. Both methods use mal.Client.NewRequest to create API
requests.

This is probably related to golang/go#18117
and it happened because when using http.NewRequest (inside
mal.Client.NewRequest) with GET, the body argument was not nil as it
should. This commit ensures that nil body is properly passed to
http.NewRequest when mal.Client.NewRequest has no data to encode.

mal/mal.go

@@ -5,6 +5,7 @@ import (
 	"encoding/xml"
 	"errors"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"net/http"
 	"net/url"
@@ -159,16 +160,18 @@ func (c *Client) NewRequest(method, urlStr string, data interface{}) (*http.Requ
 
 	u := c.BaseURL.ResolveReference(rel)
 
-	v := url.Values{}
+	var body io.Reader
 	if data != nil {
 		d, merr := xml.Marshal(data)
 		if merr != nil {
 			return nil, merr
 		}
+		v := url.Values{}
 		v.Set("data", string(d))
+		body = strings.NewReader(v.Encode())
 	}
 
-	req, err := http.NewRequest(method, u.String(), strings.NewReader(v.Encode()))
+	req, err := http.NewRequest(method, u.String(), body)
 	if err != nil {
 		return nil, err
 	}