revise sock attribute support

This fix allows you to get sock parameter properly:

    php_fpm_pool 'www' do
      user 'www-data'
      group 'www-data'
      # custom sock path
      sock '/var/run/php_fpm_www_foobar.sock'
    end

    php_fpm_pool 'hoge' do
      user 'www-data'
      group 'www-data'
      # Use default sock path based on name
    end

    t1 = resource('php_fpm_pool[www]')
    t1.sock == '/var/run/php_fpm_www_foobar.sock' #=> true

    t2 = resource('php_fpm_pool[hoge]')
    t2.sock == '/var/run/php5-fpm-hoge.sock' #=> true