Update AUTH=PLAIN to be a little closer to RFC4616

* Add authzid support
* must not contain NULL chars
* improve rdoc

lib/net/imap/authenticators/plain.rb

@@ -4,15 +4,30 @@
 #
 # See RFC4616[https://tools.ietf.org/html/rfc4616] for the specification.
 class Net::IMAP::PlainAuthenticator
+
   def process(data)
-    return "\0#{@user}\0#{@password}"
+    return "#@authzid\0#@username\0#@password"
   end
 
+  NULL = -"\0".b
+
   private
 
-  def initialize(user, password)
-    @user = user
+  # +username+ is the authentication identity, the identity whose +password+ is
+  # used.  +username+ is referred to as +authcid+ by
+  # RFC4616[https://tools.ietf.org/html/rfc4616].
+  #
+  # +authzid+ is the authorization identity (identity to act as).  It can
+  # usually be left blank. When +authzid+ is left blank (nil or empty string)
+  # the server will derive an identity from the credentials and use that as the
+  # authorization identity.
+  def initialize(username, password, authzid: nil)
+    raise ArgumentError, "username contains NULL" if username&.include?(NULL)
+    raise ArgumentError, "password contains NULL" if password&.include?(NULL)
+    raise ArgumentError, "authzid  contains NULL" if authzid&.include?(NULL)
+    @username = username
     @password = password
+    @authzid  = authzid
   end
 
   Net::IMAP.add_authenticator "PLAIN", self

test/net/imap/test_imap_authenticators.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require "net/imap"
+require "test/unit"
+
+class IMAPAuthenticatorsTest < Test::Unit::TestCase
+
+  PLAIN = Net::IMAP::PlainAuthenticator
+
+  def test_plain
+    assert_equal("\0authc\0passwd",
+                 PLAIN.new("authc", "passwd").process(nil))
+    assert_equal("authz\0user\0pass",
+                 PLAIN.new("user", "pass", authzid: "authz").process(nil))
+  end
+
+  def test_plain_no_null_chars
+    assert_raise(ArgumentError) { PLAIN.new("bad\0user", "pass") }
+    assert_raise(ArgumentError) { PLAIN.new("user", "bad\0pass") }
+    assert_raise(ArgumentError) { PLAIN.new("u", "p", authzid: "bad\0authz") }
+  end
+
+end