Fix frozen string literal compatibility issues

NB: the Sass tests are just skipped because that gem won't ever
be compatible.

lib/sprockets.rb

@@ -105,7 +105,7 @@ module Sprockets
   register_bundle_processor 'application/javascript', Bundle
   register_bundle_processor 'text/css', Bundle
 
-  register_bundle_metadata_reducer '*/*', :data, proc { "" }, :concat
+  register_bundle_metadata_reducer '*/*', :data, proc { "" }, :+
   register_bundle_metadata_reducer 'application/javascript', :data, proc { "" }, Utils.method(:concat_javascript_sources)
   register_bundle_metadata_reducer '*/*', :links, :+
 

lib/sprockets/directive_processor.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: false
+
 require 'set'
 require 'shellwords'
 

lib/sprockets/server.rb

@@ -251,11 +251,11 @@ def cache_headers(env, etag)
         # If the request url contains a fingerprint, set a long
         # expires on the response
         if path_fingerprint(env["PATH_INFO"])
-          headers["Cache-Control"] << ", max-age=31536000"
+          headers["Cache-Control"] += ", max-age=31536000"
 
         # Otherwise set `must-revalidate` since the asset could be modified.
         else
-          headers["Cache-Control"] << ", must-revalidate"
+          headers["Cache-Control"] += ", must-revalidate"
           headers["Vary"] = "Accept-Encoding"
         end
 

lib/sprockets/utils.rb

@@ -102,6 +102,7 @@ def string_end_with_semicolon?(str)
     #
     # Returns buf String.
     def concat_javascript_sources(buf, source)
+      buf = +buf
       if source.bytesize > 0
         buf << source
 

test/test_asset.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: false
+
 require "sprockets_test"
 
 module AssetTests

test/test_digest_utils.rb

@@ -35,7 +35,7 @@ def test_digest
     assert_equal "62427aa539a0b78e90fd710dc0e15f2960771ba44214b5d41d4a93a8b2940a38", hexdigest({"foo" => "baz"})
     assert_equal "b6054efd9929004bdd0a1c09eb2d12961325396da749143def3e9a4050aa703e", hexdigest([[:foo, 1]])
     assert_equal "79a19ffe41ecebd5dc35e95363e0b4aa79b139a22bc650384df57eb09842f099", hexdigest([{:foo => 1}])
-    assert_equal "94ee40cca7c2c6d2a134033d2f5a31c488cad5d3dcc61a3dbb5e2a858635874b", hexdigest("foo".force_encoding('UTF-8').encoding)
+    assert_equal "94ee40cca7c2c6d2a134033d2f5a31c488cad5d3dcc61a3dbb5e2a858635874b", hexdigest(Encoding::UTF_8)
 
     assert_raises(TypeError) do
       digest(Object.new)

test/test_encoding.rb

@@ -1,4 +1,5 @@
 # encoding: utf-8
+# frozen_string_literal: false
 require "sprockets_test"
 
 class AssetEncodingTest < Sprockets::TestCase

test/test_sass.rb

@@ -1,5 +1,7 @@
 require 'sprockets_test'
 
+return if "test".frozen? # The sass gem isn't frozen string literal compatible
+
 silence_warnings do
   require 'sass'
 end