Force default dims for downloadImage to null

Author: nicolaskruchten

src/snapshot/download.js

@@ -31,6 +31,8 @@ function downloadImage(gd, opts) {
 
     opts = opts || {};
     opts.format = opts.format || 'png';
+    opts.width = opts.width || null;
+    opts.height = opts.height || null;
     opts.imageDataOnly = true;
 
     return new Promise(function(resolve, reject) {