[ruby/rdoc] Add frozen string literal support

https://github.com/ruby/rdoc/commit/daac9d0471

lib/rdoc/erb_partial.rb

@@ -12,7 +12,7 @@ class RDoc::ERBPartial < ERB
   def set_eoutvar compiler, eoutvar = '_erbout'
     super
 
-    compiler.pre_cmd = ["#{eoutvar} ||= ''"]
+    compiler.pre_cmd = ["#{eoutvar} ||= +''"]
   end
 
 end