Sets the function to be called when a site is requesting authentication. The URL requested and the security realm from the server are passed in the arguments [url] and [realm].
The function returns a [Future] which should complete when the
authentication has been resolved. If credentials cannot be
provided the [Future] should complete with false
. If
credentials are available the function should add these using
[addCredentials] before completing the [Future] with the value
true
.
If the [Future] completes with true the request will be retried using the updated credentials. Otherwise response processing will continue normally.