[XFA] Send URLs as strings, rather than objects (issue 1773)

Given that `URL`s aren't supported by the structured clone algorithm, see https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm, the document in issue 1773 will cause the browser to throw `DataCloneError: The object could not be cloned.`-errors and nothing will render.
To fix this, we'll instead simply send the stringified version of the `URL` to prevent these errors from occuring.

src/core/xfa/template.js

@@ -2257,7 +2257,7 @@ class Image extends StringObject {
     };
 
     if (this.href) {
-      html.attributes.src = new URL(this.href);
+      html.attributes.src = new URL(this.href).href;
       return html;
     }