Fix pr_draft? GitHub Method

[rogerluan]

Description

This PR resolves #1338

My best guess is that there was a change in GitHub's API functionality and the mergeable state now doesn't return "draft" when the PR is a PR. In fact, now there's this dedicated API, so we should just be using it anyway 😊

Tests

Dangerfile:

if defined?(github) and github.pr_draft?
  message("Test: skipping early")
  return
else
if defined?(github)
  is_github_defined = true
else
  is_github_defined = false
end
if is_github_defined
  is_draft_pr = github.pr_draft?
  if github.pr_json['draft']
      message("github.pr_json['draft'] = true")
  end
end
message("Test: not skipping. is_github_defined  = #{is_github_defined}, is_draft_pr = #{is_draft_pr}")
message("#{github.pr_json['draft']}")
message("#{github.pr_json.inspect}")

Before

After

[orta]

Perfect answer IMO