unassign milestone from issue

Fixes https://github.com/octokit/octokit.net/issues/516

- [x] helper function to convert an issue into an issue update
- [x] failing test to clear the milestone from an issue
- [x] fix the bug
Ugh, it sucks that we have to duplicate this logic. I wonder if we can convince @prabirshrestha to add more properties to `GetDelegate`.