Use runnable command as DUMMY_PAGER

Author: nobu

test/rdoc/test_rdoc_ri_driver.rb

@@ -38,7 +38,12 @@ def teardown
     super
   end
 
-  DUMMY_PAGER = ":;\n"
+  case RUBY_PLATFORM
+  when /mswin|mingw/
+    DUMMY_PAGER = "type nul"
+  else
+    DUMMY_PAGER = "true"
+  end
 
   def with_dummy_pager
     pager_env, ENV['RI_PAGER'] = ENV['RI_PAGER'], DUMMY_PAGER