title: NSURLCredential password occasionally returns no password
body: <p>The Apple documentation for NSURLCredential says that it may not always return a password. But other than the obvious reason that no password was stored with credential, is there any reason that a default credential permanently stored in NSURLCredentialStorage would randomly return no password?</p>
  
  <p>The odd thing is some percentage of the time it works fine. But occasionally reading the password property on a default credential returns nil. The credential object appears to be valid in other respects, but the password property just returns nil.</p>
  
  <p>The password reading is occurring in a thread that isn't the main thread, so I was wondering if there is some threading limitation, though I couldn't find anything in the doc.</p>
  
  <p>Is it possible that we need to retry getting the password after some period of time or something?</p>