Add testable request builder (#162)

* Add testable request builder

* improve tests

image.go

@@ -45,14 +45,8 @@ 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 = c.marshaller.marshal(request)
-	if err != nil {
-		return
-	}
-
 	urlSuffix := "/images/generations"
-	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.fullURL(urlSuffix), bytes.NewBuffer(reqBytes))
+	req, err := c.requestBuilder.build(ctx, http.MethodPost, c.fullURL(urlSuffix), request)
 	if err != nil {
 		return
 	}