title: Parse destroyed session after trying to save PFUser
body: <p><code>Parse Server</code> destroyed user session in <code>iOS</code> (<code>Parse</code> version 1.13.0) and user become unauthorized after first successful <code>PFUser</code>object's saving. Next save (any update of current user) cause a error, <code>session</code> on <code>Parse server</code> for current user disappeared  and <code>PFUser.currentUser()</code> become nil:</p>
  
  <pre><code>[Error]: cannot modify user 84Yl7S2mNE (Code: 206, Version: 1.13.0)
  </code></pre>
  
  <p>I am desperate already. How to resolve this error ?
  I am using <code>Parse server</code> on <code>Heroku</code></p>
  
  <pre><code>let configuration = ParseClientConfiguration { 
    $0.applicationId = "*************"
    $0.clientKey = "unused"
    $0.server = "http://**********.herokuapp.com/parse"
    $0.localDatastoreEnabled = false
  }
  Parse.initializeWithConfiguration(configuration)
  </code></pre>
  
  <p>Tried to disable LocalDataStore - no effect</p>