title: FileTransfer plugin, download not starting on iOS, no errors on callbacks
body: <p>I’m trying to create an Ionic 3 App that downloads an image from internet and stores it in the device file system.
  On an Android device all is working fine, on iOS the call to FileTransferObject.download(…) seems to hang doing nothing; I get no error callback and nothing is downloaded.
  I created a sample repo you can check out at: <a href="https://github.com/andreabarani/TestFileTransferPlugin" rel="nofollow noreferrer">https://github.com/andreabarani/TestFileTransferPlugin</a>
  You will find 2 branches in it:</p>
  
  <ul>
  <li>‘<strong>master</strong>’ using version 1.6.3 of the plugin</li>
  <li>‘<strong>file-transfer-plugin-1.7.0</strong>’ using version 1.7.0</li>
  </ul>
  
  <p>Both versions produce same results on iOS while 1.7.0 makes Android build fail.</p>
  
  <p>Any help is very appreciated, tnx in advance
  Andrea</p>