🔇 Fix ruby 2.7 keyword argument warning

There is _some_ value in testing that it works even without this change.
But the main purpose of the test is simply testing the conversion from a
hash parameter.  Forcing the kwargs to be empty is fine.

Other than backports, maybe, this is the last ruby 2.7 concession I'll
be making before we raise the minimum ruby requirement.  😄

test/net/imap/test_deprecated_client_options.rb

@@ -24,7 +24,7 @@ class InitializeTests < DeprecatedClientOptionsTest
 
     test "Convert obsolete options hash to keywords" do
       run_fake_server_in_thread do |server|
-        with_client(server.host, {port: server.port, ssl: false}) do |client|
+        with_client(server.host, {port: server.port, ssl: false}, **{}) do |client|
           assert_equal server.host, client.host
           assert_equal server.port, client.port
           assert_equal false, client.ssl_ctx_params