connection:didReceiveAuthenticationChallenge is deprecated

[deepiksdev]

Produces warning:

WADocumentDownloader.m:130:15: Null passed to a callee that requires a non-null argument

cc @libreliodev @avittecoqDAJ

[deepiksdev]

We can still keep connection:didReceiveAuthenticationChallenge, since the documentation specifies that:
" If connection:willSendRequestForAuthenticationChallenge: is not implemented, the older, deprecated methods connection:canAuthenticateAgainstProtectionSpace:, connection:didReceiveAuthenticationChallenge:, and connection:didCancelAuthenticationChallenge: are called instead."