Do not require positional args for #authenticate

This API is a little bit confusing, IMO.  But it does preserve backward
compatibility, while allowing authenticators that don't allow positional
parameters to work without crashing.  But, authenticators that require
only one parameter—or more than three—will still be inaccessible.

lib/net/smtp.rb

@@ -878,17 +878,28 @@ def open_message_stream(from_addr, *to_addrs, &block)   # :yield: stream
 
     DEFAULT_AUTH_TYPE = :plain
 
+    # call-seq:
+    #   authenticate(authtype = DEFAULT_AUTH_TYPE, **, &)
+    #   authenticate(username, secret, authtype = DEFAULT_AUTH_TYPE, **, &)
+    #
     # Authenticates with the server, using the "AUTH" command.
     #
     # +authtype+ is the name of a SASL authentication mechanism.
     #
     # All arguments—other than +authtype+—are forwarded to the authenticator.
     # Different authenticators may interpret the +username+ and +secret+
     # arguments differently.
-    def authenticate(username, secret, authtype = DEFAULT_AUTH_TYPE, **kwargs, &block)
-      check_auth_args authtype, username, secret, **kwargs
+    def authenticate(*args, **kwargs, &block)
+      case args.length
+      when 1, 3 then authtype = args.pop
+      when (4..)
+        raise ArgumentError, "wrong number of arguments " \
+                             "(given %d, expected 0..3)" % [args.length]
+      end
+      authtype ||= DEFAULT_AUTH_TYPE
+      check_auth_args authtype, *args, **kwargs
       authenticator = Authenticator.auth_class(authtype).new(self)
-      authenticator.auth(username, secret, **kwargs, &block)
+      authenticator.auth(*args, **kwargs, &block)
     end
 
     private