Make #authenticate delegate to #auth, sometimes

Attempting to retain backward compatibility with the v0.4.0.

lib/net/smtp.rb

@@ -874,11 +874,36 @@ def open_message_stream(from_addr, *to_addrs, &block)   # :yield: stream
 
     DEFAULT_AUTH_TYPE = :plain
 
-    # Deprecated: use #auth instead.
-    def authenticate(user, secret, authtype = DEFAULT_AUTH_TYPE)
-      # warn "DEPRECATED: use Net::SMTP#auth instead"
-      authenticator = Authenticator.auth_class(authtype).new(self)
-      critical { authenticator.auth(user, secret) }
+    # call-seq:
+    #   authenticate(**kwargs, &block)
+    #   authenticate(authtype, **kwargs, &block)
+    #   authenticate(username, secret, authtype = DEFAULT_AUTH_TYPE)
+    #
+    # If zero or one positional arguments are provided, all arguments are
+    # forwarded #auth.
+    #
+    # If two or three positional arguments are provided, they are +username+,
+    # +secret+, and an optional +authtype+.  Keyword parameters cannot be
+    # used with this form.  The +username+ and +secret+ arguments may be
+    # interpreted differently, based on +authtype+.  This form supports
+    # Authenticator subclasses, for backward compatibility.
+    def authenticate(*args, **kwargs, &block)
+      case args.length
+      when 0 then auth(**kwargs, &block)
+      when 1 then auth(args.first, **kwargs, &block)
+      when 2..3
+        username, secret, authtype = *args
+        if kwargs.any?
+          kwargs = merge_auth_params(username, secret, authtype, kwargs)
+          auth(**kwargs, &block)
+        else
+          authenticator = Authenticator.auth_class(authtype).new(self)
+          critical { authenticator.auth(username, secret) }
+        end
+      else
+        raise ArgumentError, "wrong number of arguments " \
+                             "(given %d, expected 1..3)" % [args.length]
+      end
     end
 
     # call-seq: