r/github_branch_protection: check invalid users

[shihanng]

Hi, this is PR an attempt to fix #95. As reported in the issue the GitHUB V3 API does not report error when invalid user login are submitted to replace user restrictions of protected branch. It seems that the API ignores the invalid user and update the list only with the valid ones. It also return the updated list as part of the response.

$ curl -i -d '["shihanng", "shihan_ng"]' -X PUT -H "Authorization: token $GITHUB_TOKEN" -H "Content-Type: application/json" https://api.github.com/repos/shihanng-org/test/branches/master/protection/restrictions/users
HTTP/1.1 200 OK
Date: Wed, 03 Oct 2018 02:30:48 GMT
...

[
  {
    "login": "shihanng",
    ...
    "type": "User",
    "site_admin": false
  }
]

This PR includes a way to check the resulting users list from the response and compare it with the user input in order to report the error properly.

Please let me know what you think. Thank you.

[tracypholmes]

Hi @shihanng! Thanks for this PR. LGTM 👍