Add #auth method for SASL authentication

Although `#authenticate` can be updated to make username and secret
_both_ optional, by placing the mechanism last and making it optional,
it's not possible to use an authenticator with a _single_ positional
parameter or with more than two positional parameters.  By placing
`type` first among positional parameters or as a keyword argument, we
avoid this problem.

lib/net/smtp.rb

@@ -898,6 +898,22 @@ def authenticate(*args, **kwargs, &block)
         raise ArgumentError, "wrong number of arguments " \
                              "(given %d, expected 0..3)" % [args.length]
       end
+      auth(authtype, *args, **kwargs, &block)
+    end
+
+    # call-seq:
+    #   auth(type = DEFAULT_AUTH_TYPE, ...)
+    #   auth(type: DEFAULT_AUTH_TYPE, **kwargs, &block)
+    #
+    # All arguments besides +mechanism+ are forwarded directly to the
+    # authenticator.  Alternatively, +mechanism+ can be provided by the +type+
+    # keyword parameter.  Positional parameters cannot be used with +type+.
+    #
+    # Different authenticators take different options, but common options
+    # include +authcid+ for authentication identity, +authzid+ for authorization
+    # identity, +username+ for either "authentication identity" or
+    # "authorization identity" depending on the +mechanism+, and +password+.
+    def auth(authtype = DEFAULT_AUTH_TYPE, *args, **kwargs, &block)
       authtype, args, kwargs = check_auth_args authtype, *args, **kwargs
       authenticator = Authenticator.auth_class(authtype).new(self)
       authenticator.auth(*args, **kwargs, &block)

test/net/smtp/test_smtp.rb

@@ -110,6 +110,21 @@ def test_auth_plain
       smtp = Net::SMTP.start 'localhost', server.port
       assert smtp.authenticate("account", "password", :plain).success?
       assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
+
+      server = FakeServer.start(auth: 'plain')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth("PLAIN", "account", "password").success?
+      assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
+
+      server = FakeServer.start(auth: 'plain')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth(type: "PLAIN", username: "account", secret: "password").success?
+      assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
+
+      server = FakeServer.start(auth: 'plain')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth("PLAIN", username: "account", password: "password").success?
+      assert_equal "AUTH PLAIN AGFjY291bnQAcGFzc3dvcmQ=\r\n", server.commands.last
     end
 
     def test_unsucessful_auth_plain
@@ -120,10 +135,20 @@ def test_unsucessful_auth_plain
       assert_equal "535", err.response.status
     end
 
+    def test_auth_cram_md5
+      server = FakeServer.start(auth: 'CRAM-MD5')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth(:cram_md5, "account", password: "password").success?
+    end
+
     def test_auth_login
       server = FakeServer.start(auth: 'login')
       smtp = Net::SMTP.start 'localhost', server.port
       assert smtp.authenticate("account", "password", :login).success?
+
+      server = FakeServer.start(auth: 'login')
+      smtp = Net::SMTP.start 'localhost', server.port
+      assert smtp.auth("LOGIN", username: "account", secret: "password").success?
     end
 
     def test_unsucessful_auth_login