`baseHref` is always truthy here, the way plugin.js passes it (`encodeURIComponent(editor.config.baseHref)`).

The query string parameter either ends up containing a valid base href (if one is passed to CKEditor) or the string "undefined" if `editor.config.baseHref` is `undefined`.

We should either check for "undefined" here, or get plugin.js to pass a falsey value (empty string).
Otherwise we end up settings the href attribute of the base tag to "undefined". From my experiments, this doesn't cause any problems, but we should fix it anyway.