title: Error in path with django-storages with dropbox
body: <p>This is my settings code</p>
  
  <pre><code>DEFAULT_FILE_STORAGE = 'storages.backends.dropbox.DropBoxStorage'
  DROPBOX_OAUTH2_TOKEN = "my token"
  DROPBOX_ROOT_PATH = "Apps/Djangoproject"
  </code></pre>
  
  <p>The error i am getting while uploading a image:</p>
  
  <pre><code>ValidationError: did not match pattern '(/(.|[\r\n])|id:.)|(rev:[0-9a-f]{9,})|(ns:[0-9]+(/.*)?)'
  how to solve this issue?
  </code></pre>
  
  <p>Anyone know how to solve this issue?</p>