Update Sprockets::Utils#duplicable? for Ruby 2.4+

This is very similar to rails/rails@2cb8558.

Author: maclover7

lib/sprockets/utils.rb

@@ -14,11 +14,15 @@ module Utils
     #
     # Returns false if .dup would raise a TypeError, otherwise true.
     def duplicable?(obj)
-      case obj
-      when NilClass, FalseClass, TrueClass, Symbol, Numeric
-        false
-      else
+      if RUBY_VERSION >= "2.4.0"
         true
+      else
+        case obj
+        when NilClass, FalseClass, TrueClass, Symbol, Numeric
+          false
+        else
+          true
+        end
       end
     end
 

test/test_utils.rb

@@ -4,7 +4,24 @@
 class TestUtils < MiniTest::Test
   include Sprockets::Utils
 
-  def test_duplicable
+  def test_duplicable_pre_ruby_2_4
+    skip if RUBY_VERSION >= "2.4"
+
+    objs = [nil, true, false, 1, "foo", :foo, [], {}]
+    objs.each do |obj|
+      begin
+        obj.dup
+      rescue TypeError
+        refute duplicable?(obj), "can't dup: #{obj.inspect}"
+      else
+        assert duplicable?(obj), "can dup: #{obj.inspect}"
+      end
+    end
+  end
+
+  def test_duplicable_post_ruby_2_4
+    skip if RUBY_VERSION < "2.4"
+
     objs = [nil, true, false, 1, "foo", :foo, [], {}]
     objs.each do |obj|
       begin