Net::SMTP.start arguments are keyword arguments

The helo argument is not important, but the helo argument must be
specified to specify the user and secret arguments.
If helo, user, secret, and authtype arguments are keyword arguments,
it is not necessary to specify the helo argument.

lib/net/smtp.rb

@@ -146,8 +146,8 @@ class SMTPUnsupportedCommand < ProtocolError
   # The SMTP server will judge whether it should send or reject
   # the SMTP session by inspecting the HELO domain.
   #
-  #     Net::SMTP.start('your.smtp.server', 25,
-  #                     'mail.from.domain') { |smtp| ... }
+  #     Net::SMTP.start('your.smtp.server', 25
+  #                     helo: 'mail.from.domain') { |smtp| ... }
   #
   # === SMTP Authentication
   #
@@ -157,15 +157,15 @@ class SMTPUnsupportedCommand < ProtocolError
   # SMTP.start/SMTP#start.
   #
   #     # PLAIN
-  #     Net::SMTP.start('your.smtp.server', 25, 'mail.from.domain',
-  #                     'Your Account', 'Your Password', :plain)
+  #     Net::SMTP.start('your.smtp.server', 25
+  #                     user: 'Your Account', secret: 'Your Password', authtype: :plain)
   #     # LOGIN
-  #     Net::SMTP.start('your.smtp.server', 25, 'mail.from.domain',
-  #                     'Your Account', 'Your Password', :login)
+  #     Net::SMTP.start('your.smtp.server', 25
+  #                     user: 'Your Account', secret: 'Your Password', authtype: :login)
   #
   #     # CRAM MD5
-  #     Net::SMTP.start('your.smtp.server', 25, 'mail.from.domain',
-  #                     'Your Account', 'Your Password', :cram_md5)
+  #     Net::SMTP.start('your.smtp.server', 25
+  #                     user: 'Your Account', secret: 'Your Password', authtype: :cram_md5)
   #
   class SMTP < Protocol
 
@@ -400,12 +400,16 @@ def debug_output=(arg)
     # SMTP session control
     #
 
+    #
+    # :call-seq:
+    #  start(address, port = nil, helo: 'localhost', user: nil, secret: nil, authtype: nil) { |smtp| ... }
+    #  start(address, port = nil, helo = 'localhost', user = nil, secret = nil, authtype = nil) { |smtp| ... }
     #
     # Creates a new Net::SMTP object and connects to the server.
     #
     # This method is equivalent to:
     #
-    #   Net::SMTP.new(address, port).start(helo_domain, account, password, authtype)
+    #   Net::SMTP.new(address, port).start(helo: helo_domain, user: account, secret: password, authtype: authtype)
     #
     # === Example
     #
@@ -449,17 +453,26 @@ def debug_output=(arg)
     # * Net::ReadTimeout
     # * IOError
     #
-    def SMTP.start(address, port = nil, helo = 'localhost',
-                   user = nil, secret = nil, authtype = nil,
-                   &block)   # :yield: smtp
-      new(address, port).start(helo, user, secret, authtype, &block)
+    def SMTP.start(address, port = nil, *args, helo: nil,
+                   user: nil, secret: nil, password: nil, authtype: nil,
+                   &block)
+      raise ArgumentError, "wrong number of arguments (given #{args.size + 1}, expected 1..6)" if args.size > 4
+      helo ||= args[0] || 'localhost'
+      user ||= args[1]
+      secret ||= password || args[2]
+      authtype ||= args[3]
+      new(address, port).start(helo: helo, user: user, secret: secret, authtype: authtype, &block)
     end
 
     # +true+ if the SMTP session has been started.
     def started?
       @started
     end
 
+    #
+    # :call-seq:
+    #  start(helo: 'localhost', user: nil, secret: nil, authtype: nil) { |smtp| ... }
+    #  start(helo = 'localhost', user = nil, secret = nil, authtype = nil) { |smtp| ... }
     #
     # Opens a TCP connection and starts the SMTP session.
     #
@@ -487,7 +500,7 @@ def started?
     #
     #     require 'net/smtp'
     #     smtp = Net::SMTP.new('smtp.mail.server', 25)
-    #     smtp.start(helo_domain, account, password, authtype) do |smtp|
+    #     smtp.start(helo: helo_domain, user: account, secret: password, authtype: authtype) do |smtp|
     #       smtp.send_message msgstr, 'from@example.com', ['dest@example.com']
     #     end
     #
@@ -511,8 +524,13 @@ def started?
     # * Net::ReadTimeout
     # * IOError
     #
-    def start(helo = 'localhost',
-              user = nil, secret = nil, authtype = nil)   # :yield: smtp
+    def start(*args, helo: nil,
+              user: nil, secret: nil, password: nil, authtype: nil)
+      raise ArgumentError, "wrong number of arguments (given #{args.size}, expected 0..4)" if args.size > 4
+      helo ||= args[0] || 'localhost'
+      user ||= args[1]
+      secret ||= password || args[2]
+      authtype ||= args[3]
       if block_given?
         begin
           do_start helo, user, secret, authtype