add rdoc_installed? and ri_installed?

Author: mterada1228

lib/rdoc/rubygems_hook.rb

@@ -318,5 +318,13 @@ def self.load_rdoc
     def self.rdoc_version
       RubyGemsHook.rdoc_version
     end
+
+    def rdoc_installed?
+      RubyGemsHook.new(@spec).rdoc_installed?
+    end
+
+    def ri_installed?
+      RubyGemsHook.new(@spec).ri_installed?
+    end
   end
 end