Fix fragment caching in mixed encoding scenario

actionpack/lib/action_controller/caching/fragments.rb

@@ -39,9 +39,9 @@ def fragment_for(buffer, name = {}, options = nil, &block) #:nodoc:
           if cache = read_fragment(name, options)
             buffer.safe_concat(cache.html_safe)
           else
-            pos = buffer.length
+            pos = buffer.bytesize
             block.call
-            write_fragment(name, buffer[pos..-1], options)
+            write_fragment(name, buffer.byteslice(pos..-1), options)
           end
         else
           block.call

actionpack/test/controller/caching_test.rb

@@ -622,6 +622,19 @@ def test_fragment_for
     assert_equal 'generated till now -> fragment content', buffer
   end
 
+  def test_fragment_for_bytesize
+    buffer = "\xC4\x8D"
+    buffer.force_encoding('ASCII-8BIT')
+
+    @controller.fragment_for(buffer, 'bytesize') do
+      buffer.force_encoding('UTF-8')
+      buffer << "abc"
+    end
+
+    assert_equal Encoding::UTF_8, buffer.encoding
+    assert_equal "abc", @store.read('views/bytesize')
+  end
+
   def test_html_safety
     assert_nil @store.read('views/name')
     content = 'value'.html_safe