$ua->get_basic_credentials(realm,url)
Returns the list containing the username and password for
the given realm and url when authentication is required by the server. This function is usually
called internally by request. This method becomes useful when creating a subclass of LWP::UserAgent
with its own version of get_basic_credentials. From there, you can rewrite
get_basic_credentials
to do more flexible things, like asking the user for the account information, or referring
to authentication information in a file. All you need to do is return a list,
where the first element is a username and the second element is a password.