add testable json marshaller (#161)

image.go

@@ -3,7 +3,6 @@ package openai
 import (
 	"bytes"
 	"context"
-	"encoding/json"
 	"io"
 	"mime/multipart"
 	"net/http"
@@ -47,7 +46,7 @@ type ImageResponseDataInner struct {
 // CreateImage - API call to create an image. This is the main endpoint of the DALL-E API.
 func (c *Client) CreateImage(ctx context.Context, request ImageRequest) (response ImageResponse, err error) {
 	var reqBytes []byte
-	reqBytes, err = json.Marshal(request)
+	reqBytes, err = c.marshaller.marshal(request)
 	if err != nil {
 		return
 	}