[ruby/rdoc] Prefer require 'cgi/util' instead of require 'cgi'

RDoc is using only CGI.escape, escapeHTML, and unescape.
We don't have to load the whole source code of cgi gem.

d096222cc2

lib/rdoc/method_attr.rb

@@ -289,7 +289,7 @@ class RDoc::MethodAttr < RDoc::CodeObject
   # HTML id-friendly method/attribute name
 
   def html_name
-    require 'cgi'
+    require 'cgi/util'
 
     CGI.escape(@name.gsub('-', '-2D')).gsub('%','-').sub(/^-/, '')
   end