✨ Add very basic SASL-IR support to #authenticate

WIP: No tests.  And I'd rather handle this automatically (by reading
cached capabilities).

lib/net/imap.rb

@@ -520,11 +520,17 @@ def starttls(options = {}, verify = true)
     #
     # See +Net::IMAP::Authenticators+ for more information on plugging in your
     # own authenticator.
-    def authenticate(auth_type, ...)
-      authenticator = self.class.authenticator(auth_type, ...)
-      send_command("AUTHENTICATE", auth_type) do |resp|
+    def authenticate(auth_type, *creds, sasl_ir: false, **props, &callback)
+      authenticator = self.class.authenticator(auth_type,
+                                               *creds, **props, &callback)
+      cmdargs = ["AUTHENTICATE", auth_type]
+      if sasl_ir && sasl.respond_to?(:initial_response?) && sasl.initial_response?
+        data = authenticator.process(nil)
+        cmdargs << [data].pack("m0")
+      end
+      send_command(*cmdargs) do |resp|
         if resp.instance_of?(ContinuationRequest)
-          data = authenticator.process(resp.data.text.unpack("m")[0])
+          data = authenticator.process(resp.data.text.unpack1("m"))
           s = [data].pack("m0")
           send_string_data(s)
           put_string(CRLF)