new URLSearchParams([[key1, val1], [key2, val2]]) is not working

```js
new url.URLSearchParams([['param1', 'val1'], ['...', '...']])+'';
```
This should output `"param1=val1&...=..."`

But it outputs `'param1%2Cval1%2C...%2C...='`

The spec describes it here https://url.spec.whatwg.org/#urlsearchparams, the constructor should accept either a query-string, array of [key, value]'s or plain-object

Node version tested 7.9.0