Fix the "Backslashes in Included Equations" Bug

See
  http://golem.ph.utexas.edu/instiki/revision/Known+Bugs?rev=3#IncludedEqs

Who knew that

      @pre_rendered = String.new(self)

was not the same as

      @pre_rendered = String.new
      @pre_rendered = self

?

lib/wiki_content.rb

@@ -177,7 +177,8 @@ class WikiContent < String
   def pre_render!
     unless @pre_rendered
       @chunks_by_type[Include].each{|chunk| chunk.unmask }
-      @pre_rendered = String.new(self)
+      @pre_rendered = String.new
+      @pre_rendered = self
     end
     @pre_rendered
   end