Fix error in remove_entry

When `LANG=C`, dir is `UTF-8` and `base` is 'ASCII-8BIT` in `FileUtils::Entry_#join`.
So `Encoding::CompatibilityError` occurred and files are not removed.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/arch/ruby-master/log/20200611T060002Z.fail.html.gz
```
  1) Error:
WEBrick::TestFileHandler#test_cjk_in_path:
Errno::ENOTEMPTY: Directory not empty @ dir_s_rmdir - /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/tmp/???20200611-1887828-3nn72a
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:1460:in `rmdir'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:1460:in `block in remove_dir1'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:1471:in `platform_support'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:1459:in `remove_dir1'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:1452:in `remove'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:780:in `block in remove_entry'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:1509:in `ensure in postorder_traverse'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:1509:in `postorder_traverse'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/fileutils.rb:778:in `remove_entry'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/lib/tmpdir.rb:97:in `mktmpdir'
    /home/chkbuild/chkbuild/tmp/build/20200611T060002Z/ruby/test/webrick/test_filehandler.rb:292:in `test_cjk_in_path'
```

lib/fileutils.rb

@@ -1290,7 +1290,7 @@ def door?
 
     def entries
       opts = {}
-      opts[:encoding] = ::Encoding::UTF_8 if fu_windows?
+      opts[:encoding] = path.encoding
 
       files = if Dir.respond_to?(:children)
         Dir.children(path, **opts)

test/fileutils/test_fileutils.rb

@@ -741,6 +741,17 @@ def test_rm_r_pathname
     assert_file_not_exist 'tmp/tmpdir3'
   end
 
+  def test_remove_entry_cjk_path
+    dir = "tmpdir\u3042"
+    my_rm_rf dir
+
+    Dir.mkdir dir
+    File.write("#{dir}/\u3042.txt", "test_remove_entry_cjk_path")
+
+    remove_entry dir
+    assert_file_not_exist dir
+  end
+
   def test_remove_entry_secure
     check_singleton :remove_entry_secure