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

WIP: No tests.  And I'd prefer that it works automatically (by reading
cached capabilities).

lib/net/imap.rb

@@ -1037,11 +1037,17 @@ def starttls(options = {}, verify = true)
     #      imap.login username, password
     #    end
     #
-    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]
+      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)