Fix sync_patch's use of draft field

[Gabriella439]

The map returned by Jason.decode uses strings for keys and not atoms:

>>> {:ok, json} = Jason.decode(~s({ "draft": true }))
{:ok, %{"draft" => true}}
>>> json
%{"draft" => true}

… which meant that pr[:draft] would always return nil and so bors-ng would never detect that a PR was in the draft state:

>>> json[:draft]
nil
>>> json["draft"]
true

The fix is to check for pr["draft"], which fixes the problem.

This wasn't caught by the tests because the tests create a map with atom keys, which didn't trigger this problem.