Fix jQuery #14535: Always setDocument() after iframe unload

```
   raw     gz Sizes
 57353  17341 dist/sizzle.js
 18335   7060 dist/sizzle.min.js

   raw     gz Compared to master @ 3c3c5f20f8289f4e943d11d93f5541af1b072667
  +192    +51 dist/sizzle.js
   +63    +25 dist/sizzle.min.js
```

:cry: