title: arc diff disable Test Plan 'differential.require-test-plan-field' not working
body: <p>Trying to disable the required "Test Plan: " filed on <code>arc diff</code>.
   Tried the suggestions in: <a href="https://stackoverflow.com/questions/20598026/how-do-i-disable-test-plan-enforcement-in-phabricator">How do I disable “test plan” enforcement in Phabricator?</a></p>
  
  <p>So my <code>.arcconfig</code> contains:
  <code>
      "differential.require-test-plan-field" : false, 
      "differential.show-test-plan-field" :  false,
      "base" : "git:merge-base(origin/master), arc:prompt"
  </code></p>
  
  <p>But when I run <code>arc diff --create</code> not only is the "Test Plan" field there.
  If I try to submit the commit it throws an exception, saying that the test plan is required.</p>