Add frozen string literal support

Author: deivid-rodriguez

.github/workflows/macos.yml

@@ -12,4 +12,4 @@ jobs:
         gem install bundler --no-document
         bundle install
     - name: Run test
-      run: rake
+      run: rake && RUBYOPT=--enable-frozen_string_literal rake

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