support ruby 2.4+ frozen string literals

.travis.yml

@@ -5,8 +5,15 @@ language: ruby
 before_install:
   - gem install bundler
 
-rvm:
-  - 1.9.3
-  - 2.0.0
-  - 2.3.1
-  - ruby-head
+matrix:
+  include:
+    - os: linux
+      rvm: 1.9.3
+    - os: linux
+      rvm: 2.0.0
+    - os: linux
+      rvm: 2.3.0
+    - os: linux
+      rvm: ruby-head
+      env:
+        - TEST_RUBYOPT_FROZEN_STRING_LITERAL=1

Rakefile

@@ -8,4 +8,9 @@ Bundler::GemHelper.install_tasks
 
 Rake::TestTask.new
 
+if ENV['TEST_RUBYOPT_FROZEN_STRING_LITERAL'] # see .travis.yml
+  ENV['RUBYOPT'] = "--enable-frozen-string-literal --debug=frozen-string-literal"
+  puts "enabling frozen string literals"
+end
+
 task :default => [:test]

lib/sanitize/css.rb

@@ -255,7 +255,7 @@ def property!(prop)
     return nil unless @config[:properties].include?(name)
 
     nodes          = prop[:children].dup
-    combined_value = ''
+    combined_value = String.new
 
     nodes.each do |child|
       value = child[:value]

test/test_sanitize_css.rb

@@ -181,13 +181,13 @@
 
     describe '#tree!' do
       it 'should sanitize a Crass CSS parse tree' do
-        tree = Crass.parse("@import url(foo.css);\n" <<
+        tree = Crass.parse(String.new("@import url(foo.css);\n") <<
           ".foo { background: #fff; font: 16pt 'Comic Sans MS'; }\n" <<
           "#bar { top: 125px; background: green; }")
 
         @custom.tree!(tree).must_be_same_as tree
 
-        Crass::Parser.stringify(tree).must_equal "\n" <<
+        Crass::Parser.stringify(tree).must_equal String.new("\n") <<
             ".foo { background: #fff;  }\n" <<
             "#bar {  background: green; }"
       end

test/test_unicode.rb

@@ -84,7 +84,7 @@
     end
 
     it 'should strip language tag code point characters' do
-      str = 'a'
+      str = String.new 'a'
       (0xE0000..0xE007F).each {|n| str << [n].pack('U') }
       str << 'b'