✨ Add very basic SASL-IR support to #authenticate [🚧 WIP]

WIP: No tests, so it's probably broken.
And I'd like the default `:sasl_ir` value to be configurable.

lib/net/imap.rb

@@ -1053,14 +1053,26 @@ def starttls(options = {}, verify = true)
     # If the TaggedResponse to #authenticate includes updated capabilities, they
     # will be cached.
     #
-    def authenticate(mechanism, ...)
-      authenticator = self.class.authenticator(mechanism, ...)
-      send_command("AUTHENTICATE", mechanism) do |resp|
+    def authenticate(mechanism,
+                     *creds,
+                     sasl_ir: false,
+                     **props,
+                     &callback)
+      authenticator = self.class.authenticator(mechanism,
+                                               *creds,
+                                               **props,
+                                               &callback)
+      cmdargs = ["AUTHENTICATE", mechanism]
+      sasl_ir = capable?("SASL-IR") if [nil, :auto].include?(sasl_ir)
+      if sasl_ir && SASL.initial_response?(authenticator)
+        response = authenticator.process(nil)
+        cmdargs << [response].pack("m0")
+      end
+      send_command(*cmdargs) do |resp|
         if resp.instance_of?(ContinuationRequest)
-          data = authenticator.process(resp.data.text.unpack("m")[0])
-          s = [data].pack("m0")
-          send_string_data(s)
-          put_string(CRLF)
+          challenge = resp.data.text.unpack1("m")
+          response  = authenticator.process(challenge)
+          send_string_data([response].pack("m0") + CRLF)
         end
       end
         .tap { @capabilities = capabilities_from_resp_code _1 }

lib/net/imap/sasl.rb

@@ -39,6 +39,10 @@ def saslprep(string, **opts)
         Net::IMAP::StringPrep::SASLprep.saslprep(string, **opts)
       end
 
+      def initial_response?(mechanism)
+        mechanism.respond_to?(:initial_response?) && mechanism.initial_response?
+      end
+
     end
   end