title: Xam.Plugins.Settings by James Montemagno doesn't seem to save my settings
body: <p>I'm trying to use James Montemagno's settings plugin in a Xamarin forms app, but each time I restart my app, the settings go back to the defaults, not showing the changes I made to them during the last run of the app. </p>
  
  <p>The AddOrUpdateValue and other methods have a filename parameter, which seems to be optional. Or is it required? I tried it with a filename, but it still doesn't seem to save the settings.</p>
  
  <p>Is there something I need to do to "save" the settings? I must be missing something...</p>