title: Error when accessing keychain with Firebase iOS 10
body: <p>I'm having issues with keychain on iOS 10 devices. Failed to retrieve saved users, only happens with iOS 10 users, authentication is with Firebase. This is the error:</p>
  
  <blockquote>
    <p>Error loading saved user when starting up: Error Domain=FIRAuthErrorDomain Code=17995 "An error occurred when accessing the keychain. The @c NSLocalizedFailureReasonErrorKey field in the @c NSError.userInfo dictionary will contain more information about the error encountered" UserInfo={NSLocalizedDescription=An error occurred when accessing the keychain. The @c NSLocalizedFailureReasonErrorKey field in the @c NSError.userInfo dictionary will contain more information about the error encountered, error_name=ERROR_KEYCHAIN_ERROR, NSLocalizedFailureReason=SecItemCopyMatching (0)}</p>
  </blockquote>