Fix missing method in test

Author: eizengan

spec/slack/web/api/mixins/conversations_spec.rb

@@ -9,6 +9,10 @@
   let(:klass) do
     Class.new do
       include Slack::Web::Api::Mixins::Conversations
+
+      def conversations_id_page_size
+        100
+      end
     end
   end