URL#searchParams#delete removes "?"

```js
const {URL} = require("url");

const url = new URL("http://host/?param");
url.searchParams.delete("param");
url.href;
//-> http://host/
```
It should produce `http://host/?` [according to @TimothyGu](https://github.com/jsdom/whatwg-url/issues/97#issuecomment-312401510).