add keep aspect ration when resize with no option

Author: funatsu

src/Lib/ImageTransform.php

@@ -154,7 +154,9 @@ protected function thumbnailFit(Image $image, $width, $height)
  */
  protected function thumbnailResize(Image $image, $width, $height)
  {
- return $image->resize($width, $height);
+ return $image->resize($width, $height, function ($constraint) {
+ $constraint->aspectRatio();
+ });
  }
 
  /**