[selection-api] The node argument of collapse and setPosition should be nullable (#64)

Passing null should cause it to behave like `removeAllRanges`. This is the behavior of WebKit and Blink, and an attempt to align with the spec in Blink failed with many reports of breakage:
https://code.google.com/p/chromium/issues/detail?id=395318

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/64