title: PayPal fetchDeviceReferenceTokenWithAppID returns empty
body: <p>To Integrate PayPal MECL, I did below steps.</p>
  
  <ol>
  <li>Added Library. I added MECL Library(libPayPalEC.a, PayPal.h), libxml2.dylib into the project.</li>
  <li>Implemented DeviceReferenceTokenDelegate and added delegate methods.</li>
  <li><p>Called below method by a button tapped.</p>
  
  <p>[[PayPal getPayPalInst] fetchDeviceReferenceTokenWithAppID:@"APP-80W284485P519543T" forEnvironment:ENV_SANDBOX withDelegate:self];</p></li>
  <li><p>Then called method by delegate 'receivedDeviceReferenceToken:'</p></li>
  </ol>
  
  <p>Problem is in step 4. Received token is always empty.</p>
  
  <p>I just followed manual and that's all.</p>
  
  <p>Did I something wrong?</p>