Follow symlinks.

This issue was originally reported against JRuby here: https://github.com/jruby/jruby/issues/1647

I can reproduce it with MRI 1.9.3-p484 and MRI 2.1.1-p76.

```Ruby
require 'tmpdir'
require 'find'
require 'set'
require 'rspec'

describe 'Dir.entries' do
  it 'follows symlinks' do
    Dir.mktmpdir('jruby-file-find-test') do |fn|
      FileUtils.cd(fn) do
        # Create real dir with file.
        FileUtils.mkdir_p('dir')
        FileUtils.touch('dir/foo.txt')

        # Create symlink.
        File.symlink('dir', 'dir-link')

        # Find files in symlinked dir.
        filenames = Set.new
        Dir.entries('dir-link/') do |entry|
          filenames << entry
        end

        expected_filenames = Set.new(['dir-link/', 'dir-link/foo.txt'])
        filenames.should == expected_filenames
      end
    end
  end
end
```

The above test *should* pass.