Fix Dir.chdir nested with multiple absolute paths.

The previous implementation used File.join(*paths), which blindly concatenates path components and should not be used for path resolution.  As a result, the following has unexpected behavior:

    # EXPECTED
    #
    #   Dir.pwd => "/path/to/here"
    #   FakeFS::FileSystem.normalize_path('.') => "/path/to/here"
    #
    # ACTUAL
    #
    #   Dir.pwd => ""
    #   FakeFS::FileSystem.normalize_path('.') => "/path/to/here/path/to/here"
    #
    FileUtils.mkdir_p('/path/to/here')
    Dir.chdir('/path/to/here') do
      Dir.chdir('/path/to/here') do
        puts "Dir.pwd => #{Dir.pwd.inspect}"
        puts "FakeFS::FileSystem.normalize_path('.') => #{FakeFS::FileSystem.normalize_path('.').inspect}"
      end
    end

The fix implemented here is to use Pathname#+ to handle path resolution when there is at least one level of directories.
Merge branch 'fix-chdir-with-absolute-paths'

* fix-chdir-with-absolute-paths:
  Fix Dir.chdir nested with multiple absolute paths.